I tried to upload a 3.0 version blend file into blend-exchange and got the message "All uploaded files must be in the blend file format". It appears to not recognize post 2.93 file formats.
I've disabled blend file validation for now, resolving this issue. See https://github.com/GiantCowFilms/Blend-Exchange/issues/56 for details.
If someone is handy with PHP and wants to take a crack at handling the new blend file format changes, I'd love a community contribution. It will probably be a little while before I can do this work myself.
I just encountered this error when trying to upload a compressed file from the question Geonodes: when creating an instance, get index from instancer
Turning off compression solved the issue.
Just tried again with the compressed file:
As of this afternoon, I was able to upload a 3.0 file, using Chrome on Windows 10. I then went to the Blend-Exchange github repository and verified that there was no change made. Glancing at the source, it looks like Blend-Exchange does very minimal checking to see if a file is really a blend file; so I suspect you ran into a server internal glitch that got reported incorrectly.
Are you still having the problem? If so, I recommend filling out their Help request.