Sometimes has-one UploadFields can get confused about whether or not there is an existing file that needs deleting. This setting lets you make a more robust has-one UploadField, where any existing file will be replaced. It more closely mimics simple single-file upload fields.
This is the companion setting to canUpload, letting you control whether existing files from the asset store can be referenced. It's particularly useful when using UploadField on the front-end.
