Beginning with IIS6.0 each static file type that you wished to serve to clients required a defined MIME type. If a MIME type wasn't defined a 404 Not Found status would be sent to the client (the substatus code of 3 is logged in the IIS logfile). Static files are those that are mapped to the IIS Static File Handler (rather than to an ISAPI extension, which are dynamic files).
Prior to SP1 if you had an extensionless file you needed to add a "wildcard" MIME mapping if you wished IIS to serve it, per KB326965. This had the side effect of enabling downloads of any type of static file you placed into your website (e.g. document.foo).
Beginning with SP1 you can now add a MIME type of "." to allow downloads of extensionless files. However this doesn't allow downloads of other non-defined filetypes e.g. the previously mentioned document.foo. To allow files with the extension .foo to be downloaded, you would need to define a MIME type for .foo (and rightly so).
David Wang alluded to this in a comment to a previous post, however this issue appears to be causing some confusion based on the number of queries around this that I see in forums. In essence, adding a MIME type for "." doesn't replace or substitute for a wildcard mapping - it just allows downloads of extensionless files. The reason we used wildcard mappings pre-SP1 was because IIS didn't have a way of natively allowing those types of files, and using the wildcard mapping was a workaround or hack.