There is a simple directive to set in the proftpd.conf (/etc/proftpd.conf) to control upload resuming; download resuming should be permitted by default. It can be set in the section of your proftpd.conf, using the AllowStoreRestart directive. An example of using it would be:
AllowStoreRestart on
You'll have to edit the appropriate template for the changes to be reflected permanently in the proftpd configuration file (of course). Also, ensure your FTP client supports resuming transfers. Most current clients should not have a problem with this.
Mike