Of course you can omit it! You originally gave the name, not the extension, but in some specific cases you need to write both extensions together. For example, if you use PHOTOSHOP to make a picture, in You must determine the extension when saving. Of course, this software provides you with an extension choice. You can make the picture the format you want. General software will not let you change the extension, such as editing. The software in question will never have the extension of the image format, unless you insist on changing it to that, but the file will not be read