Autodesk Inventor part file
From the Autodesk website:
The Autodesk® Inventor® software product line provides a comprehensive and flexible set of software for 3D mechanical design, product simulation, tooling creation, and design communication. Cost-effectively take advantage of a Digital Prototyping workflow to design and build better products in less time.
Here's an explanation from Wikipedia:
Inventor has a number of separate file formats used in conjunction to create complex designs. The most basic is the part file, .ipt, used to create single pieces of a design which do not move. Part files are combined to create complex designs in assembly files .iam. Both part fields and assembly files can be described in a schematic drawing file .idw. Complex demonstrations showing complex designs in motion as well as complex explosions of designs can be created in Inventor as a presentation file .ipn.