Hi Theo,
Most mobile browsers do not support IFrames, which are used by Mendix to start a download in the background. To work around this issue you could calculate the downloadurl of a file and show that in the standard link widget. The url of a file can be calculated using the following microflow expression:
'/file?target=window&fileID=' + $file/fileID
You can store this value in a (virtual) attribute and display it using the link widget.
I suggest creating a bug report as well.