Transferring some file types using MIME Type doesn't work?

Question

MIME types

Hello! I took myself for study, and further use of the social script. networks. When testing all its capabilities, it turned out that it uses MIME Type to determine the type of files. I thought it was a good thing, but it wasn't there. When sending files in zip, docx, pdf and a couple of other file types, the script does not send a message with a file in one of the above formats. The most interesting thing is that he understands rar, doc, and it is possible to send a file with these extensions.
What could be the problem? Can you please help to solve the problem with the work of MIME Type.
Thank you for contacting me