I actually agree: if most (by market share) MUAs handle the RFC-incorrect parameter encoding style, and a significant portion does not handle the RFC correct style, then we should support the de-facto standard rather than the official standard as the default.  I just wish Microsoft would write better software :)  If on the other hand it is only microsoft out of the big market share players that is broken, I'm not sure I'd want it to be the default.  But we could still support it optionally.

So yeah, we could have a policy control that governs which one is actually used.

So this is a feature request, and ideally should be supported by an investigation of what MUAs support what, by market share.  And there's another question: does this only affect the filename parameter, or is it all MIME parameters?  I would expect it to be the latter, but someone should check at least a few examples of that to be sure.