Endian. See 0x20746d66 in decimal. (0x20746d66 is a reversal of the bytes of 0x666d7420.)
In file formats, every byte must be in the correct place. If you casually read them in and out of memory, depending on your CPU vs the File Format, you can get the byte orders swapped.
Read up on "Big Endian" and "Little Endian". Some will on about it at great great length, but it's actually very easy, and important to understand if you ever are dealing with file formats, network traffic, interfacing to hardware, & many other places.