There is only one file type that uses the .WPI extension:
Inkling Sketch Binary File
Sketch file created by Inkling, a package, which includes a pen, receiver, and application, that captures paper sketches and converts them to digital sketches; contains a sketch made up of layers and pen strokes that can be manipulated, merged, or removed.
Inkling consists of a pen, receiver, and the Inkling Sketch Manager application. The pen and receiver pick up your movements along with the pressure level of your pen strokes, as you sketch and convert them into a digital image, which is the WPI file. The image can then be viewed and edited with Inkling Sketch Manager, which also lets you export the image to Photoshop, Illustrator, and SketchBook Pro for further editing.
NOTE: Wacom Inkling Sketch Manager is included with your purchase of the Wacom Inkling package.
Updated: March 30, 2014