I’ve seen FITS format files that were FIT, FITS, FTS and f and lower case variants thereof. Never encountered any that were not processed.
Have once come across software that seemed only equipped for 16 bit data – a histogram of 65536 bins was being used.
Theres some fun for anyone using Python, the Astropy library discourages the writing of 16 bit FITS files and I think it defaults to 32bit.