The ascx file you have created is just a template of DownloadList control. You can get the control ID by looping through the controls collection and find a control of type DownloadList, then get its ID.
Otherwise you need to inherit from the class and get its ID or ClientID property. The ID property or ClientID property comes from the base class which is CompositeControl.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items