Documentation states, that to use tomcat sendfile functionality, developer needs to set _response_ attributes but its _request_ attributes, that must be set. Also attribute for end offset is wrong - should it be "org.apache.tomcat.sendfile.end"? I think that it would be good to mention, that its necessery to set proper response content length header.
The changes have been committed to trunk and proposed for 6.0.x
This has been fixed in 6.0.x and will be included in 6.0.17 onwards. Thanks for the report.