@mjriding. Thanks for posting.
Have you checked the FAQ? If not, please have a look at the FAQ for this particular problem on Windows: https://www.valorapps.com/faq.html?catid=3
For Windows, you have to install the fileinfo dll. If this doesn’t work for you, then you can specify an asterisk in the mime type field and use the file extension types to control the files uploaded.
As a matter of follow-up, in @frawoer’s case, his server OS was SunOS, so he needed a custom solution.
Let me know if the FAQ item helped.