Hmm after perusing that file some more I found postscript and pdf appear to be misclassified.
I think you can just about make the case for RTF being a subtype of text/plain given that you read it when you strip away the markup in a text viewer like vi. However binary formats like application/pdf and application/postscript are no more readable in a text viewer than a binary MSWord 6 document (and Word documents aren't a subtype of text/plain).
Hmm after perusing that file some more I found postscript and pdf appear to be misclassified.
I think you can just about make the case for RTF being a subtype of text/plain given that you read it when you strip away the markup in a text viewer like vi. However binary formats like application/pdf and application/ postscript are no more readable in a text viewer than a binary MSWord 6 document (and Word documents aren't a subtype of text/plain).