Hi Jean-pascal van der endt,
We've already answered to you in the other support thread, but I will share the answer here as well, for community reference.
The uploaded files are being preserved into the UploadedFiles collection only until the first postback. Since, you are doing async postback when you update the RadBinaryImage, the UploadedFiles collection is being cleared after this, because, it is expected that the files are going to be processed only ones. The good news are that, you can save the file's file name and folder inside a session object, and the access it when needed.
the Telerik team