At HTTP Proxy → MIME Type filter you can select which file MIME types are denied to be downloaded through the HTTP proxy.

To deny types you must check the box in the 'deny' column. A special "All MIME types" item exists to deny all types at once.

If you are missing any extension, you can add it through "Add MIME type" form.

Any MIME type not listed here is, by default, allowed. In future releases, this behavior may change.