It seems when you pass a file to
lpr (or pipe to it), it attempts to guess the MIME type. Sometimes, however, it guesses wrong—and then attempts to print something "interesting" at best.
Is there a way to manually override the MIME type?
The MIME type is sent in the IPP attribute
Note that if you try a document format that your CUPS server doesn't support, you'll get an error:
That error is actually coming back from the server.