From what I understand, the end of the URL string is just one of the clues the browser uses to determine the "type" of received data (https://mimesniff.spec.whatwg.org/), and the true behavior depends on the browser's specific implementation. A part of the process involves actually reading and analyzing a small portion of the received file to see if the file really is the type that the URL claims it is. For example, I started a quick python server, and made it serve the OP image, except I renamed it as a jpg file (without actually converting the image of course). When saving the picture inside the browser, Firefox correctly identifies the file as a png image: While edge incorrectly tries to save the image as a jpg image:

Regarding your "MP3" file specifically, opening it in a hex editor reveals that the actual file contents identifies itself as an M4A file, despite what the URL claims:

So, you should be good to download them any way you find convenient, and then just renaming them to the proper extension afterwards.

This behavior is because Firefox is using the MIME type of the file on the save as dialog.

Regarding your issue importing them - does renaming the ones in the ZIP to m4a not work? If the issue is mixed mp3s and m4as, you could use something like ffmpeg probe to rename them.

My tech skills are rusting horribly but I tried to briefly take a look at this...

I'm seeing the same behavior with the Firefox save-as file extension being .m4a vs the displayed URL showing .mp3.

Opening either extension in VLC and viewing the codec info (Tools->Codec Info / Ctrl+J) shows the actual encoding to be 'MPEG AAC Audio (mp4a)'.

I didn't try grabbing a file from the torrent, but if I use the media tab in Firefox 'Page Info' (Ctrl+I) to "save as" the embedded IA player file it suggests a matching .mp3 extension with the displayed address (but the codec is still mp4a).

The main thing I'm not following is what you mean by having parse errors in Mp3tag. So far I haven't seen any metadata included in either the .mp3 or .m4a downloads (I only tested the first file), they both play audio fine, and if I browse to their directory in Mp3tag (vers. 2.57) it doesn't seem to show anything unusual to me (empty metadata, no error/alert messages). What exactly is the parse error or how is it behaving differently from what's expected?

I also just wanted to say "THANK YOU!" for an awesome support post. You posted it to the right community, gave it a short but very descriptive title, gave lots of info about what you've tried and the results you've experienced -- and did so politely :)

Maybe have a read on the 2 filetypes, not sure but I think both is mp3 but m4a has more stuff in it