def get_format_conversion_session_for_type(self, source_format_type=None, target_format_type=None, proxy=None):
Gets an ``OsidSession`` associated with the text format conversion service and the given format types.

        arg:    source_format_type (osid.type.Type): the type of the
                text format
        arg:    target_format_type (osid.type.Type): the type of the
                text format
        arg:    proxy (osid.proxy.Proxy): a proxy
        return: (osid.locale.FormatConversionSession) - a
                ``FormatConversionSession``
        raise:  NullArgument - ``source_format_type,
                target_format_type`` or ``proxy`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  Unimplemented - ``supports_format_conversion()`` or
                ``supports_visible_federation()`` is ``false``
        raise:  Unsupported -
                ``supports_format_types_for_conversion(source_format_typ
                e, target_format_record_type)`` is ``false``
        *compliance: optional -- This method must be implemented if
        ``supports_format_conversion()`` and
        ``supports_visible_federation()`` are ``true``.*