Defer attachment-bearing mails to the prefetch loop on event-bus CREATE

When the realtime path inline-decrypts a brand-new mail \"and\" its
MailDetailsBlob in the same batch, it used to render the RFC 2822
envelope right there and persist it to disk so the prefetch loop has
no work left for that mail. That shortcut is correct for mails with
zero attachments — but mails carrying attachments would end up cached
with a text/html-only body and no MIME parts for the attached files,
because the event-bus handler cannot synchronously fetch and decrypt
each File blob without making the WebSocket dispatch loop sleep.

Skip the inline-render when `mail.attachments` is non-empty so the
mail keeps `has_details=0` and the prefetch sweep picks it up,
emitting a proper `multipart/mixed` envelope through the same path
as historical mails.

crates/bridge/src/event_handler.rs

@@ -331,9 +331,19 @@ async fn apply_mail_set_entry_create(
 			target_folder.imap_path,
 			if details.is_some() { ", body inline too" } else { "" },
 		);
-		let rfc2822 = details
-			.as_ref()
-			.map(|d| crate::mail::mail_to_rfc2822(&mail, Some(d), &[]));
+		// Render the inline body to an RFC 2822 envelope only when the mail
+		// has zero attachments — those need a separate blob download per
+		// File that the event-bus handler cannot do synchronously. Mails
+		// with attachments stay at `has_details=0` so the prefetch loop
+		// picks them up and emits a proper `multipart/mixed` envelope.
+		let has_attachments = !mail.attachments.is_empty();
+		let rfc2822 = details.as_ref().and_then(|d| {
+			if has_attachments {
+				None
+			} else {
+				Some(crate::mail::mail_to_rfc2822(&mail, Some(d), &[]))
+			}
+		});
 		let mut stored = StoredMail {
 			mail,
 			details: details.clone(),
@@ -342,7 +352,7 @@ async fn apply_mail_set_entry_create(
 		};
 		// Persist the body straight away so the IMAP layer can serve FETCH
 		// BODY[] without a placeholder, and the prefetch loop skips it.
-		if let (Some(eml), true) = (rfc2822.as_deref(), details.is_some()) {
+		if let Some(eml) = rfc2822.as_deref() {
 			if let Err(e) = local_store.write_eml(&mail_eid, eml) {
 				warn!("Failed to cache eml for {}: {e}", mail_eid);
 			} else if let Err(e) = local_store.mark_has_details(&mail_eid) {