How to upload an image and attach it to a change2.WTChangeRequest2

I want to upload an image and add it as an attachment to a WTChangeRequest2

WTChangeRequest2 is a ContentHolder, so I tried ContentHelper.getUploadURL(ch); to get an upload url, I uploaded a file, it said "Content Upload was a success!". But: Where is that content now? Do I need to create anApplicationData object? If yes, how do I get my uploaded file in there?

Martin