image/png

Portable Network Graphics (PNG) was devised starting in a discussion on newsgroup comp.graphics in 1995, with the first version of its specification released in 1996. The motivation for its creation was to create a free and unencumbered image format in the wake of the patent issue with GIF.

.png

image/vnd.mozilla.apng
image/x-png

Further reading

1. Internet Assigned Numbers Authority (IANA)
2. Let's Solve the File Format Problem!