def validate_attachment(req, attachment):
    """Validate an attachment after upload but before being stored
        in Trac environment.

        Must return a list of ``(field, message)`` tuples, one for
        each problem detected. ``field`` can be any of
        ``description``, ``username``, ``filename``, ``content``, or
        `None` to indicate an overall problem with the
        attachment. Therefore, a return value of ``[]`` means
        everything is OK."""
