Fix image uploads using the wrong format (#2380)

* fix(app): preserve image attachment MIME types

Image intake discarded clipboard and desktop path metadata, then guessed JPEG when a later file object had no type. Resolve MIME once at the platform boundary and require it downstream.

* fix(app): honor native picker MIME metadata

packages/app/src/composer/actions.ts

@@ -93,7 +93,7 @@ export async function pickAndPersistImages(input: {
   return await Promise.all(
     result.map(async (picked) => {
       const fileName = picked.fileName ?? null;
-      const mimeType = picked.mimeType || "image/jpeg";
+      const mimeType = picked.mimeType;
       if (picked.source.kind === "blob") {
         return await input.persister.persistFromBlob({
           blob: picked.source.blob,