^
In my opinion, the guys at JRiver should not try to solve your issue by kludging in a few additional hard coded mime types. Instead, I would like to request that MC shall do a proper implementation of the UPnP GetProtocolInfo method.
Note: the GetProtocolInfo method allows source devices (SourceProtocolInfo) and sink devices (SinkProtocolInfo) to exchange exactly between each other which transport mechanisms (e.g. HTTP Get) that they support, which mime types (e.g. audio/l16) they support, plus information flags about the supported transport mechanism (e.g. transcoded, seekable, etc.).
The current version of MC does not take into account a renderer's SinkProtocolInfo at all (indeed I am not even sure if MC actually calls the GetProtocolInfo method ??); instead it just offers a fixed choice of a) don't convert (serve file in original format), or b) convert to MP3/PCM L16/24
Nowadays pretty much every renderer gives very explicit information about what formats it can play (SinkProtocolInfo), so I would strongly recommend that MC shall call GetProtocolInfo on each renderer and use the information received as a basis to choose which format it shall deliver to the renderer concerned.
In other words, in future MC should instead of having just "don't convert" and "always convert" settings, it should have a third "auto" option that chooses the best format based on the SinkProtocolInfo of the renderer concerned.
And furthermore, in future when MC offers tracks in response to ContentDirectory:Browse or ContentDirectory:Search requests, it should include in its offer a wider choice of ProtocolInfo formats and respective Urls that the server is able to transcode to. And it should set the ProtocolInfo attribute flags in those offers to indicate what it is offering (transcoded, seekable etc.) -- (for audio, the proposed minimum offer set would include original, pcm, flac and mp3...) And this offer set should be made known (or knowable) to the control point by means of MCs SinkProtocolInfo response to an incomimg GetProtocolInfo call.
PS bob, if you want further advice on this, then I shall be delighted to help out...