A file extension - Alpha Channel image data file
What is a file? How to open a files?
File type specification:
The A file extension is associated with the Alpha Channel image data format.
The Alpha Channel is this nifty storage area in your image's file for non-visible picture elements. Think of it as your picture's own little private utility closet/filing cabinet. The .psp picture file has these extra goodies stored in its Alpha channel that can just travel around with the picture (unseen until you need them) as a part of the file.
Major items of interest that get stored in the Alpha Channel are Selections, Masks and Creator Information. We can save this stuff to the Alpha channel and then dig it back out later to reuse for editing that picture or using in our other images.