 * When displaying an edit form for a file that does not exist,
   use the return value of mimetypes.guess_type as the default
   mimetype for the file in the form. If no guessable mimetype
   is found, the default is 'text/html'.

   This behavior can be overridden by subclassing SvnWikiView
   and implementing a custom `new_file_default_mimetype_policy`
   method. The method should take a `request` argument.
