Also handle .zipx compressed format

.zipx format is produced by recent versions of WinZip when "Best method"
is chosen for the compression method. The Linuz unzip client can't
handle the .zipx format, but the 7z client can -- so use 7z to
uncompress .zipx files if anyone uploads them. (Since we accept them as
a valid file type in LexUploadCommands.php, we should also handle them
correctly on the back end.)