The FBK file is a backup for the primary database file, which uses the FDB file extension. When all else fails, a universal file viewer is the best way to open a FBK file. What is an FBK file? File used by AutoCAD Civil 3D, a BIM (Building Information Modeling) program used for civil engineering design; saved in a plain text format. What is an FBK file? The FBK file type is primarily associated with FamilyTree. A file with an extension of.