This tool uses PHP fileinfo: https://www.php.net/manual/en/intro.fileinfo.php

> The functions in this module try to guess the content type and encoding of a file
> by looking for certain magic byte sequences at specific positions within the file.
> While this is not a bullet proof approach the heuristics used do a very good job.

It is not really bulletproof, but it does the job. Everything can be manipulated
to look alike something it isn't.

To expand or reduce the allowed filetypes, edit the SELFPASTE_ALLOWED_FILETYPES string to your needs.
Again READ the README and security info!

Read more about filetypes here: https://www.iana.org/assignments/media-types/media-types.xhtml