OmniGraffle
From Just Solve the File Format Problem
OmniGraffle is MacOS software to create graphics and visuals similar to Visio[1].
File Identification
OmniGraffle saves in an XML format with the extension .graffle. Files can also be saved using compression[2] and may begin with 1F8B0800. The root of the XML uses the "plist" DTD[3]. Later versions could save a flat file or a MacOS Package containing the plist and other components[4].
Additional extensions related are .gtemplate and .gstencil.
