: Use a tool like MediaInfo to see the internal metadata. This will tell you if it's actually an H.264, HEVC, or another format disguised with a generic name.

: This is the most reliable tool for obscure file names. It can play most codecs even if the file header is slightly corrupted.

Could you tell me ? I can give you much more specific steps if I know if it's a file on your computer, a part number, or something you saw online.