Is the filename renamed?

This is a somewhat known issue with IIS.

The history: We use a third-party library to handle file uploads in K2. That library does strict checking for the type and actual content of a file based on system utils (MIME database) and a PHP module called fileinfo. If an upload fails it means that either the file you upload is corrupt or that something is mis-configured on your server and affects secure uploads.

Make sure that:
- you have the right MIME types. I suppose you edit them in IIS like so: manage.accuwebhosting.com/knowledgebase/2443/How-to-set-MIME-types-for-web-fonts-in-IIS.html
- the fileinfo PHP module is installed

Let me know.