I get error 307 when trying to load a new update.tar.gz file.

307 means a bad file extension. The first thing to check is that it is FOR SURE a tar.gz. Often people will have extensions turn off in windows and end up with a file called tar.gz.gz.

A 307 can also be caused by other things . For example, certain hardware will report 307 if the hardware has the wrong hardware type (780 on 450 for example). The best way to diagnose this is usually to give tech services a call and have them look at the configuration. From there, they can usually help decide if its just the file name or something else.