First of all, thanks VERY much to btb for starting this entire xword project and for coming up with some excellent modifcations!
So you mean like when you use the File Manager to browse a directory and "open" a .puz file? I think that has something to do with associating an extension with a MIME type, but the documentation leaves some important details out (like how to read the new .xml specification file, or even where to put it).