When uploading a file (jpeg) via a form in IE7 I am seeing a mime-type of "image/pjpeg" instead of "image/jpeg" (as I see in Firefox or similar). It's easy enough to work around this problem, but I'm just wondering why IE7 is behaving differently in this case?
I believe it's because the JPEG you're working with is a progressive JPEG, which has a different mime-type.
IE can upload all .jpeg files as its hack MIME type 'image/pjpeg' regardless of whether or not they're actually progressive.
Don't rely on the MIME type supplied in a file upload, there is every chance it could be wrong for a variety of reasons including this one.
Because Microsoft likes to violate standards. There's no such MIME type as image/pjpeg. See for yourself: http://www.iana.org/assignments/media-types/media-types.xhtml#image
The correct MIME type for JPEG is image/jpeg, progressive or not.
There is same problem with png:
IE7: image/x-png Normal browsers: image/png
;)
来源：https://stackoverflow.com/questions/115705/why-does-ie7-specify-a-mime-type-of-image-pjpeg-rather-than-just-image-jpeg