We can make the following assumptions about the relationship between .uasset and .upayload files: 1) The .uasset may be submitted to perforce without the .upayload (if the payload is unmodified) 2) If the payload is modified then the .uasset and .upayload file must be submitted at the same time. 3) The caller has already checked the existing .upayload file (if any) to see if it contains the payload that they are looking for.