FULL PRODUCT VERSION :
java version "1.8.0_60"
Java(TM) SE Runtime Environment (build 1.8.0_60-b27)
Java HotSpot(TM) 64-Bit Server VM (build 25.60-b23, mixed mode)

A DESCRIPTION OF THE PROBLEM :
For some JPEG images the method java.net.URLConnection#guessContentTypeFromStream will return the MIME type image/jpg, which is not a valid, registered IANA mime type for JPEG images.

For other images, the correct MIME type (image/jpeg) is returned.

It could be open for debate how invalid image/jpg truly is, but here's the IANA list:
http://www.iana.org/assignments/media-types/media-types.xhtml

Regardless of the validity of the MIME type, I could not find any information on *why* Java will return two different MIME types for JPEGs.

Sadly I cannot provide the image that triggered the relevant detection code (it's from a customer), but after some googling, it seems like the relevant JPEG APP14 header is an application specific header, in this case from Adobe (see http://www.sno.phy.queensu.ca/~phil/exiftool/TagNames/JPEG.html).


EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The function should return image/jpeg for all JPEG images, not just some.
ACTUAL -
The function returned image/jpg for some JPEG images encoded with Adobe software.

REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
Manually replacing image/jpg in the result with image/jpeg.