Though probably do transmit a "Content-Disposition: inline; filename=foo.jpg" header. Otherwise, the OSes that infer media type from extension will get confused when the user goes to save the file and it appears extensionless.

(That might be called a browser bug, actually; it'd sure be nice if browsers on these OSes could take the observed media type and tack on an extension to extensionless files on Save As to ensure they'll be viewed by the OS to have that same media type. Or it might be called an OS bug: why the heck don't filesystems have a required media-type metadata field for files yet!? ;)