At HTTP Proxy → Extensions filter you can choose which file extensions are disallowed to be downloaded through the HTTP proxy.

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

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

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