How does the following sound.

We keep the default whitelist and replace allow_html_file with
two options:
  
  # Roundup has a whitelist of mime types that it will serve. If a
  # file's mime type is in the whitelist, it will be served using the
  # mime type. If the file type is not in the whitelist, the file will
  # be served with type application/octet-stream.
  # If you want to override the whitelist, set this to no and set the
  # mime types in serve_mime_types.
  use_default_whitelist = yes

  # A comma separated list of mime types that will be served by roundup.
  # If use_default_whitelist is yes, these will be added to the default
  # whitelist. Otherwise they will be the only types that are not
  # served as application/octet-stream.
  serve_mime_types = 


Thoughts?