Surface mail attachments over IMAP as multipart/mixed parts

When the prefetch loop loads a mail's body, also load every entity in
`mail.attachments`, decrypt the blob data with the new SDK helper and
embed each attachment as a base64 part of a `multipart/mixed` RFC 2822
message. Thunderbird now renders PDFs / images / etc. inline rather
than showing a body-only mail with no hint anything was attached.

* `mail_to_rfc2822` now takes a slice of (TutanotaFile, &[u8]) and emits
  a multi-part envelope when it's non-empty; the simple text/html case
  is unchanged. The boundary is derived from the mail's IdTuple so the
  cached .eml.enc bytes stay stable across rewrites.
* `MailBackend::load_attachments` is the new trait method; the
  TutaSession implementation loads each File via `crypto_client.load`
  (auto-decrypted via the file's own `_ownerEncSessionKey`) then asks
  the new `MailFacade::load_file_attachment_data` for the concatenated
  decrypted bytes.
* `prefetch_details` does a best-effort fetch — partial failure logs a
  warning and ships the body alone, on the assumption the user can
  re-open later and the next sweep will retry.

A new unit test asserts the multipart structure (boundary, body part,
attachment part with name/MIME/filename, closing boundary).

crates/bridge/src/event_handler.rs

@@ -333,7 +333,7 @@ async fn apply_mail_set_entry_create(
 		);
 		let rfc2822 = details
 			.as_ref()
-			.map(|d| crate::mail::mail_to_rfc2822(&mail, Some(d)));
+			.map(|d| crate::mail::mail_to_rfc2822(&mail, Some(d), &[]));
 		let mut stored = StoredMail {
 			mail,
 			details: details.clone(),

crates/bridge/src/imap/session.rs

@@ -392,6 +392,7 @@ impl ImapSession {
                     let rfc = mail_to_rfc2822(
                         &self.mails[idx].mail,
                         self.mails[idx].details.as_ref(),
+                        &[],
                     );
                     self.mails[idx].rfc2822 = Some(rfc);
                 } else if self.mails[idx].rfc2822.is_none() {
@@ -660,7 +661,7 @@ impl ImapSession {
 
             let details = sm.details.or(old_details);
             let rfc2822 = sm.rfc2822.or(old_rfc).unwrap_or_else(|| {
-                mail_to_rfc2822(&sm.mail, details.as_ref())
+                mail_to_rfc2822(&sm.mail, details.as_ref(), &[])
             });
 
             self.mails.push(CachedMail {
@@ -1485,6 +1486,14 @@ mod tests {
             let key = mail._id.as_ref().map(|id| id.element_id.to_string()).unwrap_or_default();
             Ok(self.details.lock().unwrap().get(&key).cloned())
         }
+        async fn load_attachments(
+            &self,
+            _mail: &Mail,
+        ) -> Result<Vec<(tutasdk::entities::generated::tutanota::TutanotaFile, Vec<u8>)>, String> {
+            // The mock has no attachment fixtures; the IMAP-session tests
+            // only exercise body/header paths.
+            Ok(Vec::new())
+        }
         async fn list_folders(&self) -> Result<Vec<FolderInfo>, String> {
             Ok(vec![inbox_folder()])
         }
@@ -1603,8 +1612,8 @@ mod tests {
 
         let backend = Arc::new(MockBackend::with_mails(vec![m1.clone(), m2.clone()]));
         let store = MailStore::new();
-        let rfc1 = crate::mail::mail_to_rfc2822(&m1, Some(&d1));
-        let rfc2 = crate::mail::mail_to_rfc2822(&m2, Some(&d2));
+        let rfc1 = crate::mail::mail_to_rfc2822(&m1, Some(&d1), &[]);
+        let rfc2 = crate::mail::mail_to_rfc2822(&m2, Some(&d2), &[]);
         store.set_folder_list(vec![inbox_folder()]).await;
         store.set_folder("inbox", vec![
             StoredMail { mail: m1, details: Some(d1), rfc2822: Some(rfc1), uid: 1 },

crates/bridge/src/mail/rfc2822.rs

@@ -1,7 +1,16 @@
 use base64::Engine;
-use tutasdk::entities::generated::tutanota::{Mail, MailAddress, MailDetails};
+use tutasdk::entities::generated::tutanota::{Mail, MailAddress, MailDetails, TutanotaFile};
 
-pub fn mail_to_rfc2822(mail: &Mail, details: Option<&MailDetails>) -> String {
+/// One decrypted attachment as it lands in the RFC 2822 we serve over IMAP:
+/// the [`TutanotaFile`] entity (for name + MIME type + cid) and the raw
+/// decrypted bytes (for the body of the part).
+pub type AttachmentPart<'a> = (&'a TutanotaFile, &'a [u8]);
+
+pub fn mail_to_rfc2822(
+    mail: &Mail,
+    details: Option<&MailDetails>,
+    attachments: &[AttachmentPart<'_>],
+) -> String {
     let mut msg = String::with_capacity(4096);
 
     let date_str = format_rfc2822_date(mail.receivedDate.as_millis());
@@ -38,10 +47,6 @@ pub fn mail_to_rfc2822(mail: &Mail, details: Option<&MailDetails>) -> String {
         msg.push_str(&format!("To: {}\r\n", format_address(first)));
     }
 
-    msg.push_str("MIME-Version: 1.0\r\n");
-    msg.push_str("Content-Type: text/html; charset=UTF-8\r\n");
-    msg.push_str("Content-Transfer-Encoding: base64\r\n");
-
     if let Some(ref id) = mail._id {
         msg.push_str(&format!(
             "Message-ID: <{}.{}@tutabridge.local>\r\n",
@@ -49,19 +54,76 @@ pub fn mail_to_rfc2822(mail: &Mail, details: Option<&MailDetails>) -> String {
         ));
     }
 
-    msg.push_str("\r\n");
+    msg.push_str("MIME-Version: 1.0\r\n");
 
     let body_text = details
         .and_then(|d| d.body.compressedText.as_deref().or(d.body.text.as_deref()))
         .unwrap_or("<p>(No body available)</p>");
 
-    let encoded = base64_encode_body(body_text.as_bytes());
-    msg.push_str(&encoded);
-    msg.push_str("\r\n");
+    if attachments.is_empty() {
+        msg.push_str("Content-Type: text/html; charset=UTF-8\r\n");
+        msg.push_str("Content-Transfer-Encoding: base64\r\n");
+        msg.push_str("\r\n");
+        msg.push_str(&base64_encode_body(body_text.as_bytes()));
+        msg.push_str("\r\n");
+    } else {
+        // The boundary is derived from the mail's element id so the same
+        // mail always produces the same MIME boundary — keeps `.eml.enc`
+        // bytes stable across rewrites.
+        let boundary = build_boundary(mail);
+        msg.push_str(&format!(
+            "Content-Type: multipart/mixed; boundary=\"{}\"\r\n",
+            boundary
+        ));
+        msg.push_str("\r\n");
+        msg.push_str("This is a multi-part message in MIME format.\r\n");
+
+        msg.push_str(&format!("--{}\r\n", boundary));
+        msg.push_str("Content-Type: text/html; charset=UTF-8\r\n");
+        msg.push_str("Content-Transfer-Encoding: base64\r\n\r\n");
+        msg.push_str(&base64_encode_body(body_text.as_bytes()));
+        msg.push_str("\r\n");
+
+        for (file, data) in attachments {
+            msg.push_str(&format!("--{}\r\n", boundary));
+            let mime = file.mimeType.as_deref().unwrap_or("application/octet-stream");
+            let name_encoded = encode_header_value(&file.name);
+            msg.push_str(&format!(
+                "Content-Type: {}; name=\"{}\"\r\n",
+                mime, name_encoded
+            ));
+            msg.push_str("Content-Transfer-Encoding: base64\r\n");
+            msg.push_str(&format!(
+                "Content-Disposition: attachment; filename=\"{}\"\r\n",
+                name_encoded
+            ));
+            if let Some(ref cid) = file.cid {
+                // Some Tuta files (inline images) carry a Content-ID — propagate
+                // it so HTML `<img src="cid:…">` references still resolve.
+                msg.push_str(&format!("Content-ID: <{}>\r\n", cid));
+            }
+            msg.push_str("\r\n");
+            msg.push_str(&base64_encode_body(data));
+            msg.push_str("\r\n");
+        }
+        msg.push_str(&format!("--{}--\r\n", boundary));
+    }
 
     msg
 }
 
+/// Build a MIME boundary that is stable for a given mail and unlikely to
+/// collide with payload bytes. Format: `=_TutaBridge_<list>_<elem>` where the
+/// ids are the mail's `IdTuple` — they contain only base64-ext characters
+/// (so safe in a Content-Type header) and uniquely identify the mail.
+fn build_boundary(mail: &Mail) -> String {
+    if let Some(ref id) = mail._id {
+        format!("=_TutaBridge_{}_{}", id.list_id, id.element_id)
+    } else {
+        "=_TutaBridge_unknown".to_owned()
+    }
+}
+
 pub(crate) fn format_address(addr: &MailAddress) -> String {
     if addr.name.is_empty() {
         addr.address.clone()
@@ -373,7 +435,7 @@ mod tests {
             _errors: Default::default(),
         };
 
-        let rfc = mail_to_rfc2822(&mail, None);
+        let rfc = mail_to_rfc2822(&mail, None, &[]);
 
         assert!(rfc.contains("Date: Wed, 25 Dec 2024 12:37:25 +0000\r\n"));
         assert!(rfc.contains("From: Alice <alice@tuta.com>\r\n"));
@@ -484,7 +546,7 @@ mod tests {
             },
         };
 
-        let rfc = mail_to_rfc2822(&mail, Some(&details));
+        let rfc = mail_to_rfc2822(&mail, Some(&details), &[]);
 
         assert!(rfc.contains("From: sender@tuta.com\r\n"));
         assert!(rfc.contains("To: Bob <bob@example.com>, charlie@example.com\r\n"));
@@ -494,4 +556,124 @@ mod tests {
             base64::engine::general_purpose::STANDARD.encode(b"<p>Hello World</p>");
         assert!(rfc.contains(&body_b64));
     }
+
+    #[test]
+    fn test_mail_to_rfc2822_with_attachment_emits_multipart() {
+        use tutasdk::date::DateTime;
+        use tutasdk::entities::generated::tutanota::{Body, Recipients, TutanotaFile};
+        use tutasdk::IdTupleGenerated;
+
+        let mail = Mail {
+            _id: Some(IdTupleGenerated::new(
+                test_id("list_att"),
+                test_id("elem_att"),
+            )),
+            _permissions: test_id("perm_att"),
+            _format: 0,
+            _ownerEncSessionKey: None,
+            subject: "With Attachment".to_string(),
+            receivedDate: DateTime::from_millis(0),
+            state: 2,
+            unread: false,
+            confidential: false,
+            replyType: 0,
+            _ownerGroup: None,
+            differentEnvelopeSender: None,
+            listUnsubscribe: false,
+            movedTime: None,
+            phishingStatus: 0,
+            authStatus: None,
+            method: 0,
+            recipientCount: 1,
+            encryptionAuthStatus: None,
+            _ownerKeyVersion: None,
+            processingState: 0,
+            processNeeded: false,
+            sendAt: None,
+            serverClassificationData: None,
+            _kdfNonce: None,
+            sender: MailAddress {
+                _id: None,
+                name: "Alice".to_string(),
+                address: "alice@tuta.com".to_string(),
+                contact: None,
+                _errors: Default::default(),
+            },
+            attachments: vec![],
+            conversationEntry: IdTupleGenerated::new(
+                test_id("conv_l"),
+                test_id("conv_e"),
+            ),
+            firstRecipient: Some(MailAddress {
+                _id: None,
+                name: "".to_string(),
+                address: "bob@example.com".to_string(),
+                contact: None,
+                _errors: Default::default(),
+            }),
+            mailDetails: None,
+            mailDetailsDraft: None,
+            bucketKey: None,
+            sets: vec![],
+            clientSpamClassifierResult: None,
+            _errors: Default::default(),
+        };
+        let details = MailDetails {
+            _id: None,
+            sentDate: DateTime::from_millis(0),
+            authStatus: 0,
+            replyTos: vec![],
+            recipients: Recipients {
+                _id: None,
+                toRecipients: vec![],
+                ccRecipients: vec![],
+                bccRecipients: vec![],
+            },
+            headers: None,
+            body: Body {
+                _id: None,
+                text: Some("<p>The body</p>".to_string()),
+                compressedText: None,
+                _errors: Default::default(),
+            },
+        };
+        let file = TutanotaFile {
+            _id: Some(IdTupleGenerated::new(
+                test_id("file_list"),
+                test_id("file_elem"),
+            )),
+            _permissions: test_id("file_perm"),
+            _format: 0,
+            _ownerEncSessionKey: None,
+            name: "doc.pdf".to_string(),
+            size: 5,
+            mimeType: Some("application/pdf".to_string()),
+            _ownerGroup: None,
+            cid: None,
+            _ownerKeyVersion: None,
+            _kdfNonce: None,
+            parent: None,
+            subFiles: None,
+            blobs: vec![],
+            _errors: Default::default(),
+        };
+        let data: &[u8] = b"PDFDA";
+        let attachments: Vec<super::AttachmentPart> = vec![(&file, data)];
+        let rfc = mail_to_rfc2822(&mail, Some(&details), &attachments);
+
+        assert!(rfc.contains("Content-Type: multipart/mixed; boundary=\"=_TutaBridge_list_att_elem_att\""));
+        assert!(rfc.contains("--=_TutaBridge_list_att_elem_att\r\n"));
+        // Body part: text/html base64
+        assert!(rfc.contains("Content-Type: text/html; charset=UTF-8\r\n"));
+        let body_b64 =
+            base64::engine::general_purpose::STANDARD.encode(b"<p>The body</p>");
+        assert!(rfc.contains(&body_b64));
+        // Attachment part
+        assert!(rfc.contains("Content-Type: application/pdf; name=\"doc.pdf\""));
+        assert!(rfc.contains("Content-Disposition: attachment; filename=\"doc.pdf\""));
+        let pdf_b64 = base64::engine::general_purpose::STANDARD.encode(data);
+        assert!(rfc.contains(&pdf_b64));
+        // Closing boundary
+        assert!(rfc.ends_with("--=_TutaBridge_list_att_elem_att--\r\n"));
+    }
 }

crates/bridge/src/sync.rs

@@ -4,7 +4,7 @@ use std::time::Duration;
 
 use log::{debug, info, warn};
 use tokio::sync::{watch, RwLock};
-use tutasdk::entities::generated::tutanota::{Mail, MailDetails};
+use tutasdk::entities::generated::tutanota::{Mail, MailDetails, TutanotaFile};
 
 use crate::mail::mail_to_rfc2822;
 use crate::store::{LocalStore, MailMetadata};
@@ -453,10 +453,10 @@ async fn load_cached_folder(
                 }
                 Some(eml)
             },
-            Ok(None) => Some(mail_to_rfc2822(&mail, None)),
+            Ok(None) => Some(mail_to_rfc2822(&mail, None, &[])),
             Err(e) => {
                 warn!("Failed to read cached eml {}: {e}", meta.element_id);
-                Some(mail_to_rfc2822(&mail, None))
+                Some(mail_to_rfc2822(&mail, None, &[]))
             },
         };
 
@@ -529,7 +529,7 @@ pub(crate) async fn sync_folder(
                 uid,
             });
         } else {
-            let rfc2822 = mail_to_rfc2822(mail, None);
+            let rfc2822 = mail_to_rfc2822(mail, None, &[]);
             updated.push(StoredMail {
                 mail: mail.clone(),
                 details: None,
@@ -611,7 +611,29 @@ async fn prefetch_details(
         let result = retry(|| backend.load_mail_details(mail)).await;
         match result {
             Ok(Some(details)) => {
-                let rfc2822 = mail_to_rfc2822(mail, Some(&details));
+                // Best-effort attachment fetch: if any blob fails to load we
+                // still ship the body — the user can re-open the mail later
+                // and the prefetch sweep will retry. Empty `mail.attachments`
+                // short-circuits inside the backend so this is free for the
+                // overwhelmingly common "no attachments" case.
+                let attachments_owned = match backend.load_attachments(mail).await {
+                    Ok(atts) => atts,
+                    Err(e) => {
+                        warn!(
+                            "Failed to load attachments for {}: {e}",
+                            mail._id
+                                .as_ref()
+                                .map(|id| id.element_id.to_string())
+                                .unwrap_or_default()
+                        );
+                        Vec::new()
+                    }
+                };
+                let attachment_refs: Vec<(&TutanotaFile, &[u8])> = attachments_owned
+                    .iter()
+                    .map(|(f, d)| (f, d.as_slice()))
+                    .collect();
+                let rfc2822 = mail_to_rfc2822(mail, Some(&details), &attachment_refs);
                 if let Some(id) = mail._id.as_ref() {
                     let eid = id.element_id.to_string();
 
@@ -632,7 +654,7 @@ async fn prefetch_details(
                 // `MailDetailsDraft` (legacy / malformed). Persist a
                 // headers-only `.eml` and mark the row done so we don't
                 // re-attempt every prefetch sweep.
-                let rfc2822 = mail_to_rfc2822(mail, None);
+                let rfc2822 = mail_to_rfc2822(mail, None, &[]);
                 if let Some(id) = mail._id.as_ref() {
                     let eid = id.element_id.to_string();
                     if let Err(e) = local_store.write_eml(&eid, &rfc2822) {

crates/bridge/src/tuta.rs

@@ -9,7 +9,7 @@ use tutasdk::bindings::rest_client::RestClient;
 use tutasdk::crypto_entity_client::CryptoEntityClient;
 use tutasdk::entities::generated::tutanota::{
     DraftCreateData, DraftData, DraftRecipient, Mail, MailBox, MailDetails, MailDetailsBlob,
-    MailSetEntry, SendDraftData, SendDraftParameters,
+    MailSetEntry, SendDraftData, SendDraftParameters, TutanotaFile,
 };
 use tutasdk::folder_system::{FolderSystem, MailSetKind};
 use tutasdk::services::generated::tutanota::{DraftService, SendDraftService};
@@ -67,6 +67,15 @@ pub trait MailBackend: Send + Sync {
         json: &str,
     ) -> Result<Option<MailDetails>, String>;
     async fn load_mail_details(&self, mail: &Mail) -> Result<Option<MailDetails>, String>;
+    /// Load and decrypt every attachment of `mail`. Returns a vector of
+    /// `(file metadata, decrypted bytes)` in the order they appear in
+    /// `mail.attachments`. Empty if the mail has no attachments. Errors are
+    /// per-mail (no partial returns): if any one attachment fails the whole
+    /// call returns `Err` so the caller can decide to retry the prefetch.
+    async fn load_attachments(
+        &self,
+        mail: &Mail,
+    ) -> Result<Vec<(TutanotaFile, Vec<u8>)>, String>;
     /// Enumerate all mail folders (system + custom, with hierarchy).
     async fn list_folders(&self) -> Result<Vec<FolderInfo>, String>;
     async fn set_unread_status(&self, mail_ids: Vec<IdTupleGenerated>, unread: bool) -> Result<(), String>;
@@ -292,6 +301,24 @@ impl TutaSession {
         }
     }
 
+    async fn load_attachments_impl(
+        &self,
+        mail: &Mail,
+    ) -> Result<Vec<(TutanotaFile, Vec<u8>)>, ApiCallError> {
+        if mail.attachments.is_empty() {
+            return Ok(Vec::new());
+        }
+        let crypto_client = self.crypto_client();
+        let mail_facade = self.logged_in.mail_facade();
+        let mut out: Vec<(TutanotaFile, Vec<u8>)> = Vec::with_capacity(mail.attachments.len());
+        for file_id in &mail.attachments {
+            let file: TutanotaFile = crypto_client.load(file_id).await?;
+            let data = mail_facade.load_file_attachment_data(&file).await?;
+            out.push((file, data));
+        }
+        Ok(out)
+    }
+
     async fn send_mail_impl(&self, msg: &ParsedMessage) -> Result<(), ApiCallError> {
         let randomizer = RandomizerFacade::from_core(rand_core::OsRng);
         let session_key: GenericAesKey = Aes256Key::generate(&randomizer).into();
@@ -403,6 +430,13 @@ impl MailBackend for TutaSession {
             .map_err(|e| format!("{e}"))
     }
 
+    async fn load_attachments(
+        &self,
+        mail: &Mail,
+    ) -> Result<Vec<(TutanotaFile, Vec<u8>)>, String> {
+        self.load_attachments_impl(mail).await.map_err(|e| format!("{e}"))
+    }
+
     async fn list_folders(&self) -> Result<Vec<FolderInfo>, String> {
         let mailbox = self.load_mailbox().await.map_err(|e| format!("{e}"))?;
         let folder_system = self.load_folders(&mailbox).await.map_err(|e| format!("{e}"))?;