The file extension used to name a BUNDLE target on the OS X and iOS.
The default value is bundle - you can also use plugin or whatever file extension is required by the host app for your bundle.
© 2000–2016 Kitware, Inc.
© 2000–2011 Insight Software Consortium
Licensed under the BSD 3-clause License.
https://cmake.org/cmake/help/v3.5/prop_tgt/BUNDLE_EXTENSION.html