I encountered this problem while saving a locally rendered image to my files.
When I click to rename the file, the extension .PNG can be easily deleted. It's even easier to make this mistake because the default file name is longer than the box, so I don't notice it was erased when I type in a better name for the file. The solution for an individual is simple, add .PNG to the file name.
A good example of a better system is the way Photoshop provides a drop down tab to choose a file type when saving an image. This makes it impossible to accidentally delete it.
