I received an email attachment, with no filename extension because of the spam filters in corporate email. I “knew” it was a legitimate file because I had just requested it from a notable researcher. Rather than bother the sender to tell me the original file extension, I determined the file type by:
which gave output
gzip compressed data, from Unix
Thus I changed the filename to be
emailedfilename.tar.gz since a gzip’d file almost always contains a tar archive, and I could extract the files.
tar is smart enough even with the wrong file extension to work, so I could have instead just done
tar -xf emailedfilename