Dunno. I had a poke around this evening -- there's some CLSID values stored in there, so I thought I'd put together a quick COM DLL and hook it in instead -- to see what kind of things happen when you attempt to download a .mp3 file, but no luck so far.

The other possible avenue is to implement a "pluggable protocol handler", but that looks scary, and is probably overkill.
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-- roger