[file] Is the MIME type 'image/jpg' the same as 'image/jpeg'?

The important thing to note here is that the mime type is not the same as the file extension. Sometimes, however, they have the same value.

https://www.iana.org/assignments/media-types/media-types.xhtml includes a list of registered Mime types, though there is nothing stopping you from making up your own, as long as you are at both the sending and the receiving end. Here is where Microsoft comes in to the picture.

Where there is a lot of confusion is the fact that operating systems have their own way of identifying file types by using the tail end of the file name, referred to as the extension. In modern operating systems, the whole name is one long string, but in more primitive operating systems, it is treated as a separate attribute.

The OS which caused the confusion is MSDOS, which had limited the extension to 3 characters. This limitation is inherited to this day in devices, such as SD cards, which still store data in the same way.

One side effect of this limitation is that some file extensions, such as .gif match their Mime Type, image/gif, while others are compromised. This includes image/jpeg whose extension is shortened to .jpg. Even in modern Windows, where the limitation is lifted, Microsoft never let the past go, and so the file extension is still the shortened version.

Given that that:

1. File Extensions are not File Types
2. Historically, some operating systems had serious file name limitations
3. Some operating systems will just go ahead and make up their own rules

The short answer is:

• Technically, there is no such thing as image/jpg, so the answer is that it is not the same as image/jpeg
• That won’t stop some operating systems and software from treating it as if it is the same

While we’re at it …

Legacy versions of Internet Explorer took the liberty of uploading jpeg files with the Mime Type of image/pjpeg, which, of course, just means more work for everybody else. They also uploaded png files as image/x-png.