Suppose you wish to prohibit WordPress attachments deletion for your authors. WordPress manages access to the Media Library items (attachments) using ‘posts’ capabilities set. Yes, the key capability to access the ‘Media Library’ is ‘upload_files’, but in order to edit uploaded image attributes a user needs ‘edit_posts’ capability, in order to delete uploaded video a user needs ‘delete_posts’ capability, etc. So there is no way to prohibit attachments deletion for users who may (and should) delete posts with WordPress default Media Library permissions model.
Pro version of User Role Editor offers a workaround – ‘Force custom post types to use their own capabilities’ option: Read more Prohibit authors attachments deletion