There is only one file type that uses the .MCAT extension:
Adobe Lightroom CC Catalog File
An MCAT file is a catalog file created by Adobe Lightroom CC, a digital photo management and editing application. It stores a catalog of information about a collection of imported images that is used to organize the images in Lightroom CC.
MCAT files are created when you import photos into Adobe Lightroom CC. The file contains references to where the photos are located, edits that have been made to the photos in Lightroom CC, and metadata that organizes the photos.
NOTE: Lightroom CC was formerly known as Photoshop Lightroom.
Updated: August 29, 2017