Revert "Use mimetypes.guess_type() to determine the content type of a Node"

Trac mimeviewer machinery already applies a few heuristics to
determine the mime type of a particular Node, when the underlying
backend does not return a precise value. I was fooled by the default
value of the option "mimeviewer.mime_map_patterns", which basically
maps "README*" to "plain/text".

This reverts commit 0a2c0baba5653a9d3647561fe9133201d28725cc.