Firefox 3.5.5, except when I absolutely am forced to use Internet Explorer, then it is IE8.
+++++
Additional Note re: Sound Files...
Even after changing file association for MID from Windows Media Player, to Musicmatch Jukebox (which cannot play it anyway), and then closing windows and doing a cold reboot, the search for MID files showed the same icon (not Musicmatch) and displayed in Tiles not Details (other Musicmatch associated files will show in details).

+++++
A couple of theories I have considered...

1. Perhaps ADS (Alternate Data Stream) information was being employed by Windows, not just the extensions, in order to determine file handling. But since the behavior is consistent, whether NTFS or FAT32, this idea needs to be discarded, does it not?

2. Perhaps Windows Explorer looks at the file more closely than just the extension, seeing in the initial information of the file what type of file it really is, regardless of what it is called, and then referring to registry information for file handling. I see no reason to discard this idea/theory yet. The question would be how the registry stores that file-handling information. Some seems to be under FileExts and some under ComDlg32
(HKEY_CURRENT_USER\Software\Microsoft\Windows\Curr entVersion\Explorer\). I am not changing any of this information at this point because I am still unclear exactly how it is being used. HKLM also has data on which program to use for opening files.