Fix stale binary image fallback for empty modified diffs and relay reads

- mobileDiffImageDataUri now distinguishes a true deletion (modified
  side absent) from a modify whose binary bytes arrived empty
  (relay/size-cap cases), returning null instead of the stale
  pre-change image
- readWorkingDiffFile passes the file path to bufferToBlob so relay
  working-tree reads can detect previewable image extensions instead
  of always reporting empty binary content
- add mobile-file-tab-doc.test.ts covering diff/image/binary/text
  resolution paths

src/relay/git-working-file-read.test.ts

@@ -35,4 +35,16 @@ describe('readWorkingDiffFile', () => {
       isBinary: true
     })
   })
+
+  it('base64-encodes a previewable image by its extension', async () => {
+    tmpDir = await mkdtemp(path.join(tmpdir(), 'relay-working-file-'))
+    const filePath = path.join(tmpDir, 'icon.png')
+    const pngBytes = Buffer.from([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a, 0x00, 0x01])
+    await writeFile(filePath, pngBytes)
+
+    await expect(readWorkingDiffFile(filePath)).resolves.toEqual({
+      content: pngBytes.toString('base64'),
+      isBinary: true
+    })
+  })
 })

src/relay/git-working-file-read.ts

@@ -16,7 +16,9 @@ export async function readWorkingDiffFile(
       return { content: '', isBinary: true }
     }
     const buffer = await readFile(absPath)
-    return bufferToBlob(buffer)
+    // Why: bufferToBlob needs the path's extension to know an image is
+    // previewable; omitting it made every relay-side binary diff empty.
+    return bufferToBlob(buffer, absPath)
   } catch {
     return { content: '', isBinary: false }
   }