I'm afraid this answer isn't 100% correct. There are ways to find out a file's type beyond looking at an extension. For example, there are lots of file formats where all of the files start with a specific sequence of bites, known as a file signature (or as "magic bytes" or "magic numbers").
You can try the file
command line tool to check that you can find out a file's format without resorting to its extension, and you can read the tool's manpage to learn how it works.
Mozilla is corrupt? How so?