http://www.fileinfo.com/extension/menc

Encrypted file created by Windows mobile devices; typically found on removable media cards; contains an encrypted version of the original file and additional header information; enables secure file storage on external storage cards.

MENC files are named with the convention FILENAME.GUID.menc, where FILENAME is the original file name and GUID is a generated key that lets an encryption filter know whether or not it can decrypt the file.