File extensions are suggestions to your computer operating system what kind of data is in the file so it knows what application to open with it. They have no special meaning besides this. As pointed out in other answers, older operating systems put hard limits on file name total length and only understood 3 character file extensions, so .jpg is the older extension format for JPEG images. They mean the same thing and if you were to change the extension to .picture then open it in Paint (or whatever your OS has) it would accomplish the same thing, since the extension is just a suggestion about what application cares about this file.
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