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

No, image/jpg is not the same as image/jpeg.

You should use image/jpeg. Only image/jpeg is recognised as the actual mime type for JPEG files.

See https://www.rfc-editor.org/rfc/rfc3745, https://www.w3.org/Graphics/JPEG/ .

Serving the incorrect Content-Type of image/jpg to IE can cause issues, see http://www.bennadel.com/blog/2609-internet-explorer-aborts-images-with-the-wrong-mime-type.htm.

tl;dr the "standards" are a hodge-podge mess; it depends who you ask!

Overall, there appears to be no MIME type image/jpg. Yet, in practice, nearly all software handles image files named "*.jpg" just fine.
This particular topic is confusing because the varying association of file name extension associated to a MIME type depends which organization created the table of file name extensions to MIME types. In other words, file name extension .jpg could be many different things.

For example, here are three "complete lists" and one RFC that with varying JPEG Image format file name extensions and the associated MIME types.

• sitepoint.com mime-types-complete-list (archived)
  • .jfif, .jfif-tbnl, .jpe, .jpeg, .jpg → image/jpeg
  • .jfif, .jpe, .jpeg, .jpg → image/pjpeg
• freeformatter.com mime-types (archived)
  • .jpeg, .jpg → image/jpeg
  • .jpeg, .jpg → image/x-citrix-jpeg
  • .pjpeg → image/pjpeg
• IANA "Media Types" (formerly known as MIME types) lists (archived)
  (this document lists "names", not "file name extensions")
  • jpg not mentioned
  • jpeg → see RFC 2045 (no mention), see RFC 2046 → image/jpeg¹³
  • JPEG → video/JPEG
  • jpeg2000 → video/jpeg2000
  • jpm → image/jpm (JPEG 2000)
  • jpx → image/jpx (JPEG 2000)
  • vnd.sealedmedia.softseal.jpg → image/vnd.sealedmedia.softseal.jpg
• RFC 3745 MIME Type Registrations for JPEG 2000 (ISO/IEC 15444)
  • .jpg file extension not mentioned
  • .jpeg file extension not mentioned
  • .jp2, .jpg2 → image/jp2 (JPEG 2000)⁹
  • .jpf, .jpx → image/jpx (JPEG 2000 ISO-JPEG2000-2 encoded image data)¹⁰
  • .mj2, .mjp2 → video/mj2 (Motion JPEG 2000)¹¹
  • .jpm, .jpgm → image/jpm (JPEG 2000 profiles and extensions)¹²

These "complete lists" and RFC do not have MIME type image/jpg! But for MIME type image/jpeg some lists do have varying file name extensions (.jpeg, .jpg, …). Other lists do not mention image/jpeg.

Also, there are different types of JPEG Image formats (e.g. Progressive JPEG Image format, JPEG 2000, etcetera) and "JPEG Extensions" that may or may not overlap in file name extension and declared MIME type.

Another confusing thing is RFC 3745 does not appear to match IANA Media Types yet the same RFC is supposed to inform the IANA Media Types document. For example, in RFC 3745 .jpf is preferred file extension for image/jpx but in IANA Media Types the name jpf is not present (and that IANA document references RFC 3745!).

Another confusing thing is IANA Media Types lists "names" but does not list "file name extensions". This is on purpose, but confuses the endeavor of mapping file name extensions to MIME types.

Another confusing thing: is it "mime", or "MIME", or "MIME type", or "mime type", or "mime/type", or "media type"⁉ 😧

The most official seeming document by IANA is surprisingly inadequate. No MIME type is registered for file extension .jpg yet there exists the odd vnd.sealedmedia.softseal.jpg. File extension.JPEG is only known as a video type while file extension .jpeg is an image type (when did lowercase and uppercase letters start mattering!?). At the same time, jpeg2000 is type video yet RFC 3745 considers JPEG 2000 an image type! The IANA list seems to cater to company-specific jpeg formats (e.g. vnd.sealedmedia.softseal.jpg).

In summary...

Because of the prior confusions, it is difficult to find an industry-accepted canonical document that maps file name extensions to MIME types, particularly for the JPEG Image File Format.

Related question "List of ALL MimeTypes on the Planet, mapped to File Extensions?".

For those it might help, I use this list as a reference to define my content-type when I have to deal with images on my app.

It says that jpg extension can be declared with Content-type : image/jpeg

There isn't any image/jpg attribute for content-type.

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.