$1- is a set of space delimited tokens, and mIRC doesn't support null tokens. It'd take a pretty fundamental (and largely non-backwards compatible) change to the scripting engine for $1- to act the way you want.
As far as $findfile() goes you can simply use $findfile().shortfn and $1- will contain the short filename and thus work correctly. You can use $longfn($1-) if/when you want to display the filename anywhere (it'll still have the consecutive spaces missing when displayed but it's better than not working at all).
