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/hooks/image-attachment-picker.native.ts

@@ -34,6 +34,8 @@ export async function normalizePickedImageAssets(
   return normalizePickedImageAssetsWith(assets, exportPickedImageAsPng);
 }
 
-export async function openImagePathsWithDesktopDialog(_dialog?: unknown): Promise<string[]> {
+export async function pickImagesWithDesktopDialog(
+  _dialog?: unknown,
+): Promise<PickedImageAttachmentInput[]> {
   throw new Error("Desktop dialog API is not available on native.");
 }