What does this phrase mean in the documentation?

Question

PHP

Hello! I am studying uploading files to the server and came across this phrase in this article :

$_FILES['userfile']['type']
The mime type of the file, in case the browser provided such information. An example would be "image/gif". This mime type is not checked on the PHP side, so don't rely on its value without checking.

How can this MIME type be checked if it is not checked on the PHP side? How to check it on the client? And how will check on the server?
Thanks in advance!

Answer 1

The documentation says that you should not rely on and use information from $_FILES['userfile']['type'] for serious checks, but you should determine the MIME type of the file yourself, for example, using the mime_content_type function