What kind of file is this?

It's the terminal output:

---
[mjb@march ~]$ gnome-music
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:43 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:44 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
20:27:44 WARNING	Error: <class 'TypeError'>, MediaArt.buffer_to_jpeg() takes exactly 4 arguments (3 given)
---

In albumartcache.py the function is called with three arguments:

   MediaArt.buffer_to_jpeg(map_info.data, mime, cache_path)

But the function is defined in "extractpixbuf.c" as

   media_art_buffer_to_jpeg (const unsigned char  *buffer,
                             size_t                len,
                             const gchar          *buffer_mime,
                             const gchar          *target,
                             GError              **error)

I mean, what is the format of the file?

Oh sorry. Standard mp3. All the covers used to show up in older versions of gnome-music

What distribution?

Up-to-date Arch Linux

See #792272, libmediaart built with meson is currently not providing correct introspection. You should report this to the arch maintainers.

The fix has been integrated in libmediaart, notify your libmediaart package maintainers.

Thanks for your report.

Thank you so much for the fix. It's much appreciated.

I will notify the Arch maintainers as soon as there is a new release of libmediaart.

(In reply to mjb from comment #9)
> Thank you so much for the fix. It's much appreciated.
> 
> I will notify the Arch maintainers as soon as there is a new release of
> libmediaart.

I would suggest you notify them now. It is easy for maintainers to apply the patch and fix the package, libmediaart does not release often.