The attachment form now contains a license agreement for uploading files:

License agreement

By uploading the attachment you agree that either you are the author of the work and release it under the CC-by license (for artwork) or ISC license (for code), or have mentioned the author and license in the message body.
Example:
Copyright © 2009 Jon Doe
License: CC-BY

You may use the checkbox for adding a standard CC-by statement to your post (for any kind of artwork i.e. graphics, music, sounds etc.). We ask everyone who uploaded one or more files so far to give us any information about the license. Otherwise the files will be deleted soon.