Tomcat read/write file permissions

[Mistery]

Hello all, I need to deploy a tomcat webapp to let users upload files to a particular vhosts domain's data subdirectory but actually I get a File Permission error.
How should I change the default catalina.policy to solve this issue ?
I tried adding the following permissions

permission java.util.PropertyPermission "java.io.tmpdir","read";
permission java.io.FilePermission "/path/to/final-destination/directory/-","write";

but it's still not working.
Any advice ?