Why is it that whenever I download a file from Yahoo Briefcase it is corrupt? Surely this is more than a coincidence?

This is a known problem with Yahoo servers and is commonly referred to as the 'extra bytes' bug. The problem occurs whenever additional bytes are tacked onto the files you wish to download rendering them inaccessible. For the files to function normally, these extra bytes have to be removed, but don't try and edit them yourself, get YBC Fix to do the job for you instead.