The above filter syntax works for predefined filters. Loadable filters are not currently used. The working definition of their syntax, which is being redefined due to it being difficult to parse, is: If the filter name contains an unquoted colon or slash, then it is interpreted as a loadable filter. If the filter name does not, it is interpreted as a predefined filter. Note that you can specify a loadable filter in the current virtual directory by prefixing its name with the sequence ./.