If I remember correctly, you cannot directly work on temporary files. I think the reason was that once a file is uploaded and nothing else is done with it, it is auto-removed from the tmp location. For that reason you are seeing the "unable to open". The file is no longer there to open it.
Instead, after the upload first move the file into a place inside your subscription where you could equally well open it with an editor, then apply the imagick functions to that file.