What is the difference between: image/x-citrix-pjpeg and image/pjpeg

Question

Some files are uploaded with a reported MIME type:

image/x-citrix-pjpeg

They are valid jpeg files and I accept them as such.

I was wondering however: why is the MIME type different?
Is there any difference in the format? or was this mimetype invented by some light bulb at citrix for no apparent reason?

Answer 1

Update:
Ok, I did some more searching and testing on this question, and it turns out they're all lying about MIME-type (never trust any info send by the client, I know).
I've checked a bunch of files with different encodings (created with libjpeg)

Official MIME type for jpeg files: image/jpeg

But some applications (most notably MS Internet Explores but also Yahoo! mail) send jpeg files as image/pjpeg

I thought I knew that pjpeg stood for 'progressive' jpeg. It turns out that progressive/standard encoding has nothing to do with it.

MS Internet explorer send out all jpeg files as pjpeg regardless of the contents of the file.

The same goes for citrix: all jpeg files send from a citrix client are reported as the image/x-citrix-pjpeg MIME type.

The files themselves are untouched (identical before and after upload). So it turns out that difference in MIME type is only an indication the software used to send the file?

Why would people invent a new MIME type if there is no differences to the file contents?