This is one of the most common mix-ups in computing, and the answer is refreshingly simple: JPEG and JPG are the exact same image format. There is no technical difference whatsoever between a file ending in .jpeg and one ending in .jpg.
Where the confusion comes from
The format is named after the Joint Photographic Experts Group, the committee that created it — hence 'JPEG'. Early versions of Windows, however, required file extensions to be exactly three characters long, which was a limitation of the older FAT file system. Since 'jpeg' has four letters, it had to be shortened to 'jpg' to work on those systems. Mac and Unix systems never had that restriction, so 'jpeg' remained common there.
Does the extension matter today
Modern operating systems handle both extensions identically, and virtually every piece of software recognizes both without issue. The only time it matters is when a specific upload form or older piece of software is coded to only accept files ending in exactly '.jpg', rejecting '.jpeg' purely due to a strict extension check rather than an actual format difference.
What to do if a form rejects your .jpeg file
Re-saving or re-exporting the same image with a .jpg extension resolves the issue instantly, since nothing about the underlying image data needs to change — only the file name.
Standardize your file extension.
Open JPEG to JPG