Append file extension to Blossom URLs when missing

Blossom servers return content-addressed URLs (/<sha256>) that may omit
the file extension. After upload, extract the extension from the source
filename and append it to the URL if not already present. This helps
clients infer the media type from the URL alone.

src/hooks/useUploadFile.ts

@@ -23,6 +23,15 @@ export function useUploadFile() {
       });
 
       const tags = await uploader.upload(file);
+
+      // If the returned URL is missing a file extension, append one from the
+      // source file name. Blossom URLs are content-addressed (`/<sha256>`) and
+      // may omit the extension. Adding it helps clients infer the media type.
+      const ext = getFileExtension(file.name);
+      if (ext) {
+        tags[0][1] = appendExtensionIfMissing(tags[0][1], ext);
+      }
+
       const url = tags[0][1];
 
       // Mirror to all other servers in the background (fire-and-forget).
@@ -42,6 +51,23 @@ export function useUploadFile() {
   });
 }
 
+/** Extract the file extension (with leading dot) from a filename, or empty string if none. */
+function getFileExtension(filename: string): string {
+  const dotIndex = filename.lastIndexOf('.');
+  if (dotIndex <= 0) return '';
+  return filename.slice(dotIndex).toLowerCase();
+}
+
+/** Append a file extension to a URL if its path doesn't already have one. */
+function appendExtensionIfMissing(urlString: string, ext: string): string {
+  const url = new URL(urlString);
+  const lastSegment = url.pathname.split('/').pop() ?? '';
+  // Check if the last path segment already contains a dot (has an extension)
+  if (lastSegment.includes('.')) return urlString;
+  url.pathname = url.pathname + ext;
+  return url.toString();
+}
+
 /** Mirror a blob to additional Blossom servers (BUD-04). */
 async function mirrorToServers(
   sourceUrl: string,