thx for the report! This will be supported in the next beta version of the WebKit plugin for Royal TSX V4.
If you're not using the beta yet you can get it here: https://www.royalapplications.com/ts/mac/features-beta
I'm happy to report that Royal TSX 4.0 has been released and includes this feature.
For more information about the major new features of Royal TSX 4.0, please head over to our major upgrade landing page.
It think I found a bug;
When I want to upload/select a file from within a Web Page the Browse screen wont show.
Click on Choose File and nothing happens.