Use temp file to save metadata for readonly file

May be worth an 'enhance' comment noting that when we have to do this temp file thing, we will not get the same rights the next time we insert the same file, which is one of the main reasons ClearShare encourages people to set the rights at the time they select the image. Eventually, we could consider building some sort of persistent map from source file path to temp file (or containing a copy of the metadata) so we could do better.