Fixed: changed file format of a default cover from .jpg to .jpeg (#636)

### Description

This should resolve issue #635

### Checklist

- [ ] Created tests which fail without the change (if possible)
- [x] All tests passing
- [ ] Extended the documentation, if necessary

app/modules/ph/endpoints_ph.py

@@ -202,7 +202,7 @@ async def get_cover(
         )
 
     return get_file_from_data(
-        default_asset="assets/images/default_cover.jpg",
+        default_asset="assets/images/default_cover.jpeg",
         directory="ph/cover",
         filename=str(paper_id),
     )