> Shall we consider adding the following:
>
> #define isc_blob_jpeg 25
> #define isc_blob_gif 26
> #define isc_blob_rtf 27
> #define isc_blob_xml -400
> #define isc_blob_html 28
> #define isc_blob_wav 29
> #define isc_blob_mp3 30

What about...
#define isc_blob_ico 31 Windows icon files
#define isc_blob_gdb 32 InterBase database file
#define isc_blob_dfm 33 Delphi Form file
#define isc_blob_bsh 34 Script file
?

> (Is MIDI a format or a protocol?).

It's a format (AFAIK).

> Some enterprising person should find a pointer to a definitive
> specification for each of the above.

In general, www.wotsit.org is a great source.

JPEG:
http://www.jpeg.org/

GIF:
(a number of different formats)
http://www.wotsit.org/search.asp?page=4&s=graphics

RTF:
http://msdn.microsoft.com/library/default.asp?URL=/library/specs/rtfspec.htm

XML:
http://www.w3.org/TR/1998/REC-xml-19980210

HTML:
(pick your flavor)
http://www.w3.org/MarkUp/
(they are now talking about XHTML - puaj!)

WAV:
http://www.wotsit.org/search.asp?page=9&s=music
(the MS descriptions are quite complete)