Mamiya MEF is a raw camera image format (or family of formats?) used by some Mamiya digital cameras.
It is sometimes known as Mamiya RAW, though this can also be a collective name for both MEF and Leaf MOS.
At least some MEF files use DNG format, or something similar to DNG. The first IFD (refer to TIFF) contains a thumbnail image, and is expected to have three sub-IFDs. The first sub-IFD contains the full-resolution image, and the others contain reduced-resolution images.