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/utils/image-attachments-from-files.test.ts

@@ -92,7 +92,7 @@ describe("collectImageFilesFromClipboardData", () => {
       ],
     });
 
-    expect(files).toEqual([imagePng]);
+    expect(files).toEqual([{ file: imagePng, mimeType: "image/png" }]);
   });
 
   it("ignores SVG clipboard files", () => {
@@ -127,7 +127,10 @@ describe("filesToImageAttachments", () => {
       type: "",
     });
 
-    const attachments = await filesToImageAttachments([pngFile, typeLessFile]);
+    const attachments = await filesToImageAttachments([
+      { file: pngFile, mimeType: "image/png" },
+      { file: typeLessFile, mimeType: "image/png" },
+    ]);
 
     expect(attachments).toEqual([
       {
@@ -141,7 +144,7 @@ describe("filesToImageAttachments", () => {
       },
       {
         id: "att-2",
-        mimeType: "image/jpeg",
+        mimeType: "image/png",
         storageType: "web-indexeddb",
         storageKey: "att-2",
         fileName: "second",
@@ -156,7 +159,7 @@ describe("filesToImageAttachments", () => {
       type: "image/png",
     });
 
-    const [attachment] = await filesToImageAttachments([large]);
+    const [attachment] = await filesToImageAttachments([{ file: large, mimeType: "image/png" }]);
 
     expect(attachment?.storageType).toBe("web-indexeddb");
     expect(attachment?.byteSize).toBe(4 * 1024 * 1024);