There is only one file type that uses the .MENC extension:
Windows Mobile Encrypted File
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.
NOTE: MENC files are designed to be decrypted by the device that created the file. Therefore, MENC files can be transferred to a PC using Windows ActiveSync or Windows Mobile Device Center, but it may not be possible to decrypt the file on the computer. Also, changing the GUID on a MENC file will disable the ability of the decryption program to properly decrypt the file.
To avoid encrypting files, go toor on the Windows Mobile device and uncheck the box that says "Encrypt files placed on storage cards."
Updated: April 8, 2010