When attempting to make the request with for action=upload with the 'file' field enabled, the JavaScript console (at least in Firefox 68) logs "TypeError: this.setIcon is not a function" and the request is not sent.
This was probably broken by rGOJU4d76a9d9fd4f: Rewrite SelectFileInputWidget as an ActionFieldLayout, which removed OO.ui.mixin.IconElement from SelectFileWidget / SelectFileInputWidget.