Preferred MIME type of S-expression in `HTTP_ACCEPT` header?

NancyFX has Content Negotiation feature that I can write additional serialization adapter for S-expression by implementing IResponseProcessor interface.

And there is a problem… S-expression’s MIME type seems to be text/plain. So the web server cannot decide actual text file type in HTTP_ACCEPT header.

Is there well known practice for deciding MIME type of text file? For example,

  • text/plain+s: adding its file extension
  • text/plain+s-expression: adding language name