File type: iTunes Cover Data Flow
These files contain the artwork for albums as well as the metadata of the songs that have artwork for their covers. These files are used and read by the iTunes software which is an Apple product, to speed up the browsing of the songs in the iTunes software in its Cover flow mode where it shows the thumbnails of the covers of the albums.
These files are saved by default by the iTunes application under the ~/Music/iTunes/Album Artwork/Local/ directory. These files are created, read, edited or modified by the iTunes software directly.
These are not meant to be opened and edited manually since it could corrupt the data in them and create problems.
The album artworks for the songs can be downloaded separately with your iTunes library by clicking on the 'Get Album Artwork' option from the Advanced menu within the iTunes application.