AtHomeInTheUniverse t1_ixtxjci wrote on November 26, 2022 at 9:03 AM

Many file formats have what is literally called a "Magic Number" at the very beginning of the file. If the program sees that magic number, they know it is that file format.

https://en.wikipedia.org/wiki/List_of_file_signatures

capilot t1_ixv2kbw wrote on November 26, 2022 at 4:31 PM

Fixed link: https://en.wikipedia.org/wiki/List_of_file_signatures

Fun fact: I've encountered jpeg files mis-labeled as ".gif" and vice-versa. A lot of software has no trouble with this at all, since it looks at the magic number. Your web browser for example.