fix(post-form): hide filename for non-file media links

Only show pasted link filenames when the URL resolves to publishable
file media, so bare paths like imgur album IDs are not shown as files.

src/lib/utils/media-link-validation-utils.ts

@@ -1,8 +1,25 @@
-import { getExpiringMediaLinkHostname } from './url-utils';
+import { getExpiringMediaLinkHostname, getPublishURLFilename } from './url-utils';
+import { getLinkMediaInfo } from './media-utils';
 
 type TranslateFn = (key: string, options?: Record<string, unknown>) => string;
 
+const PUBLISH_FILE_MEDIA_TYPES = new Set(['gif', 'image', 'video']);
+
 export const getExpiringMediaLinkAlert = (url: string, t: TranslateFn): string | null => {
   const expiringMediaLinkHostname = getExpiringMediaLinkHostname(url);
   return expiringMediaLinkHostname ? `${t('error')}: ${t('expiring_media_link_alert', { domain: expiringMediaLinkHostname })}` : null;
 };
+
+export const isPublishFileMediaType = (type: string | undefined): boolean => Boolean(type && PUBLISH_FILE_MEDIA_TYPES.has(type));
+
+export const isPublishFileMediaLink = (link: string): boolean => isPublishFileMediaType(getLinkMediaInfo(link)?.type);
+
+export const getPublishFileDisplayName = (url: string, uploadedFileName: string | null | undefined, requireFile: boolean): string | null => {
+  if (!url) {
+    return uploadedFileName || null;
+  }
+  if (requireFile && !isPublishFileMediaLink(url)) {
+    return null;
+  }
+  return getPublishURLFilename(url) || uploadedFileName || null;
+};