Multiple Upload Page - Assign metadata

1. Create an override for the upload page

2. Do forget to call base.OnLoad(e) in your overriden OnLoad page

3. Set this.ConfirmationURL parameter to absolute path of a custom layouts page in the overriden upload page Onload method.

4. Call the overriden upload page using a custom action on the list and pass the same url parameters that SP passes to an OOTB Multiple file upload page

5. On the new custom page, collect user input and using ListItem.SystemUpdate(false) to set the required metadata fields. Once the metadata is set, navigate back to list default view.