Recourse fp. Is a file pointer.
So you would say something like
$fp = fopen("file.dat","r+");
$array = mailparse_uudecode($fp);
It looks like it returns an array of some of the file specifications or something. Its undocumented and experimental.
UUEncoded is a way to encode file attachments. So the array probably returns file type, size...etc. (Just a guess) Its really designed for binary attachments.
Hope this Helps!