refactor(mobile): address code review feedback

Extract APP_SCHEME_PREFIX constant for the custom URL scheme string,
and derive the photo library fallback filename extension from the
asset's MIME type instead of always using .jpg.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

mobile/src/screens/UploadScreen.tsx

@@ -180,7 +180,9 @@ export default function UploadScreen() {
 
     if (!result.canceled && result.assets.length > 0) {
       const asset = result.assets[0];
-      const filename = asset.fileName ?? `photo_${Date.now()}.jpg`;
+      // Derive extension from MIME type so the filename matches the actual format
+      const ext = asset.mimeType?.split("/")[1]?.replace("jpeg", "jpg") ?? "jpg";
+      const filename = asset.fileName ?? `photo_${Date.now()}.${ext}`;
       await uploadFile(asset.uri, filename, asset.mimeType ?? "image/jpeg");
     }
   }