Thank you both.

If you unzip them, the file extension will tell what they are.
Check out the docs, but each emulator eats different file extensions, like Megadrive = .md, MasterSystem = .sms - this could get you some of the way I guess.

@tubadog said in Problem uploading files through retropie manager:

I know this thread is 3 years old.... but.

I had the same problem, but it was inconsistent. I wanted to add Rugrider to the Commodore 64 emulator. I was dragging and dropping and everything but no luck. I was able to drag and drop plenty of other games though.

SOLUTION:
The file extension was .D64 and was failing. I changed it to .d64 and it succeeded. Seems that the file type is case sensitive for this instance at least.

Best of luck!

Maybe worth noting for folks who may not be aware - Linux is case sensitive, unlike Windows.