What do file extensions do?

File extensions are suffixes appended to filenames that help operating systems identify which program should open a given file. They are not part of the file's actual contents and can be changed without corrupting the data. When a file is opened, the OS checks a registry of associated programs and routes the file accordingly. Beyond extensions, files also contain 'magic bytes' at the start of their binary data that identify the true file type, providing a secondary identification mechanism. Opening a file with an incompatible program can potentially destroy it, especially if that program doesn't check magic bytes.