At work we're going to be making some videos available on our website in WMV and Quicktime 7 formats. Simple enough. Well, it turns out that if you save your Quicktime file as a "M4V" file (h.264 codec I believe) that IIS6 throws a 404 File Not Found error when its requested. You can search your log files until you're blue in the face but it doesn't make sense.
Until you Google the problem, that is, when you discover that its a mimetypes problem. Yep, IIS6 handles unknown (unwanted?) mimetypes with revulsion and simply blocks the file rather than falling back to something like simply downloading the file.
So to fix it you have to load up the IIS Manager, go to the properties on your website (or the master websites properties), go to the HTTP Headers tab, click MIME Types, click New, type "m4v" as the file extension and "video/x-m4v" as the MIME type, then click OK twice then Apply and you're done.
Bit of a pain if you ask me, but at least now I know to manually configure each media filetype I intend using.