ART (ArtCAM model)
ART is the file format of ArtCAM, a 3D modeling program.
ArtCAM was originally(?) developed by Delcam. Delcam was acquired by Autodesk in 2014. Autodesk discontinued ArtCAM as of July 7, 2018. ArtCAM has since been revived under the name Carveco.
Its format doesn't seem to be documented online. Various sites claim to have downloads of files of this format, but they all seem to either be for pay or at least with a registration wall, so there's nothing easily findable to examine the format. Early versions of ArtCAM actually required a proprietary dongle in your computer to even open the files.