There is currently no way to do that through Genio patterns. The best you can do is add a validation routine to that field that will check if the extension is what you want to allow. However this will not set the open file dialog box to only select those kind of files and will also not prevent the document from being uploaded to the server. It will only prevent the record as a whole from being saved.

You will have to do use native code manual routines to achieve that effect and that depends on the target platform you are generating to.