Custom fieldtype for contour that renders file upload however unlike out of box contour one this allows regex. The reg ex can be used to restrict types of files that can be uploaded.
To use copy the dll over to your bin. In contour when adding fields you will be able to select one of type Cog Filtered upload. After adding it goto additional settings and add the following reg ex
Update the extension in brackets to whatever you like. You can add additional ones by adding |(whatever)
This regex will fire when form is submitted.