Some downloaders have an option to change the extension to match the file type. On Windows you can get the “file” command from git’s bash terminal, Cygwin, or WSL2. Linux and macOS have “file” command that inspects the first part of a file and makes a (usually correct) guess about the contents. If a download terminated early you may get a partial file and the size alone will indicate a problem, but I have also had files with blocks replaced by “NUL” values so you should look for ways to verify a checksum. When a batch of downloads fails at the same time it is possible that the downloads have been replaced by a text or html document with error messages.
