Note: this is not a tested situation (at least by myself). It worked for others and can work for you, so I found it interesting to post it here.
Upload of multiple files to a document library built from a document library template or upgraded from Sharepoint 2007 to Sharepoint 2010.
No files present in the document library after the upload.
- Files with no check in are unmanaged
- Alien metadata* fields attached to files
- In Library Settings, choose Permissions and Management and then check Manage files which have no checked in version, and bulk check in the checked out files
- Remove the alien metadata columns, check the option in previous step, and bulk check in all the checked out files