When I attempt to save a file as "picture.jpg", OS X unhelpfully informs me that the standard extension is ".jpeg". I think the bazillion .jpg files on the web beg to differ, so where does Apple get this notion that it is supposed to be ".jpeg"?
The 'standard' extension is derived from the Uniform Type Identifier ( http://en.wikipedia.org/wiki/Uniform_Type_Identifier ) which, for JPEG files is
public.jpeg. It's just a different method of identifying a file (as opposed to looking at the MIME type information).
For more information about JPEG file formats: http://en.wikipedia.org/wiki/JPEG
After a coworker approached me about his displeasure with his system's apparent insistence on JPEGs ending in .jpeg under particular circumstances, I decided to look into this issue.
Following some research, I tried to coax the public.jpeg UTI declaration stored in /System/Library/CoreServices/CoreTypes.bundle/Contents/Info.plist to favor the .jpg extension over the .jpeg extension by switching the extension order like so:
<key>public.filename-extension</key> <array> <string>jpg</string> <string>jpeg</string> <string>jpe</string> </array>
`Rebooted the computer after making the modification, but alas: the application insisting on JPEGs ending in .jpeg (saving a .jpg attachment in Microsoft Outlook using the Message > Attachments > Save... menu item, if you must know) continued to insist on the .jpeg extension. Even removing the .jpeg extension declaration (a rather drastic measure) and another reboot didn't do anything to change this. Apple's Uniform Type Identifier system appears to be a many-headed hydra that is difficult to train.
Related observations: loading the URL of an online JPEG image that ends in the .jpg extension in a web browser will result in a drag and drop to the desktop sans changed extension in both Safari (5.1.7) and Firefox (15.0.1). Google Chrome (22.0.1229.79) however does change the .jpg to a .jpeg. In the aforementioned Microsoft Outlook (14.2.4) dragging and dropping a .jpg attachment to the desktop retains the .jpg extension, but going through the Message > Attachments > Save... menu or right-clicking the attachment and choosing Save As... will insist on a JPEG image filename needing to end in a .jpeg extension.
Between the crapshoot nature of this small sampling of application behaviors and my inability to effectively tweak a public identifier locally I cut my losses and gave up.