REST Service URL with Contains Filter

Hi all, I tried to get REST service with contains in filter; however I got error and not sure if the syntax was correct.  Here is my URL:

http://sharepoint.acme.domain/sites/twf/_api/web/lists/getbytitle('Mail')/items?$select=Id,Title,LinkFilename,ServerUrl,EncodedAbsUrl,Created,ContentType,FileLeafRef&$filter=contains(ServerUrl, '/sites/twf/Mail/test')

Please help.

Thanks

Tee