Unify attachment display and enable file uploads on all platforms (#1501)

Attachment pills were drawn by four separate style sets, so files,
images, GitHub items, and reviews rendered at different heights in the
same tray. Move the frame, the two-line label, and the image thumbnail
into shared components so every attachment lines up at one height,
across both the composer and sent messages.

Sent-message images now open the lightbox, and file uploads work on
iOS and Android via expo-document-picker instead of being gated to
desktop.

packages/app/src/attachments/file-types.ts

@@ -69,6 +69,11 @@ export function getFileExtension(path: string): string {
   return normalizedPath.slice(extensionIndex).toLowerCase();
 }
 
+export function getFileTypeLabel(path: string): string | null {
+  const extension = getFileExtension(path).slice(1);
+  return extension ? extension.toUpperCase() : null;
+}
+
 export function getMimeTypeFromPath(path: string): string {
   return MIME_TYPE_BY_EXTENSION[getFileExtension(path)] ?? "application/octet-stream";
 }