@wwannotificationicon.png

Portable Network Graphics image Reference

A .png file contains a raster image in the Portable Network Graphics format. The compression is lossless, thus the image quality does not decrease. The format supports full color and an alpha channel for transparency. PNG is an open standard, and all common browsers and image tools can read it. Icons, screenshots, and web graphics frequently use this format.

A file extension is only a part of the file name. It does not give a guarantee about the format or the content of a file. But on Windows, the extension selects the program that opens the file.