We should allow users to cancel/remove an upload on the "Details" step too.
Perhaps you're uploading a series of files (location maps for a whole country, or pronunciation audio recordings for multiple words), and realize one of them already exists only when UploadWizard warns you of a file name conflict. Or maybe one of the files is rejected by other checks, you can't figure out how to fix it, and this blocks you from uploading all the other files.
Requested at https://commons.wikimedia.org/wiki/Commons:Upload_Wizard_feedback#Can.27t_stop_upload_at_.22Describe.22_screen