[FFmpeg-user] Changing the Quicktime "Format" visible in Quicktime Player

[Marcus Johnson]

you can't, it's hardcoded, you yourself can in your program that you're
calling the libX functions with make a function that writes "H.264" to the
encoder field after it's finished, it wouldn't really be that hard to do.

or you could create a script to replace the bytes at X to Y with string Z,
just make sure the file works properly because it may have to have the
header field size updated, dig into the h.264 header specs to find out how
specifically to do this.