|Message:||Another user reported this issue and it seemed to be the browser version. He was running OSX. It seems the browser was not properly sending the file bytes to the server. He tried another browser and it all worked OK.
Also be aware that the .pmx file is actually a zip file, so if you have a windows machine with winzip installed, it may get decompressed during download. To check, open the pmx file with winzip and view the enclosed file, it should look like an xml document.
Let us know how you get on.