to work is usually a matter of fixing the file extension or repairing a corrupted header. Start with VLC, and if that fails, move to a repair utility.
If you have a whole archive of these files (e.g., archivefhdjufe567 , archivefhdjufe568 , etc.), you don't want to fix them one by one. Using Deobfuscation Tools archivefhdjufe568 3mp4 work