Codec (short for Compressor/Decompressor) is the "wrapper" in which a file is contained. The computer must have the proper codec reader to be able to open the file. The computer does not know a file is an image unless it knows that the file is a .jpg, .psd, .png, etc. Then, if it has the proper codec for that file type, it can open the image.

This is true of all file types, be they a .docx or a .ac3.