But relying on them to tell the OS what type a file is, or allowing them to change the extension, isn't good either. lena.jpg doesn't become lena.pdf by changing the last 3 letters of the filename..
It gets really complicated when you get into overlapping file types, like with ISOBMFF. An .mp4 can also, simultaneously, be a valid .3gp because those are profiles of ISOBMFF. On the other extreme, JPEG is secretly two different incompatible formats (JFIF and Exif), and a video file with a different codec in the same container, or even a different track layout, might as well be a different format.