Why do I get Media Type "image/ipeg" when uploading a PDF?

Question

I am getting this strange "image/ipeg" media type when uploading with FF (33.0.2 on Mac OS X 10.9.4), instead of "application/pdf." Viewing the request sent using Developer Tools, I get the following headers:
Content-Disposition: form-data; name="test"; filename="test.pdf"
Content-Type: image/ipeg
Looking at mimeTypes.rdf in my that profile, I see
<RDF:Description RDF:about="urn:mimetype:image/ipeg"
NC:value="image/ipeg"
NC:editable="true"
NC:fileExtensions="pdf"
NC:description="Document">
<NC:handlerProp RDF:resource="urn:mimetype:handler:image/ipeg"/>
When creating a new profile and uploading a PDF, I correctly get "application/pdf", and sure enough, mimeTypes.rdf states:
<RDF:Description RDF:about="urn:mimetype:application/pdf"
NC:value="application/pdf">
<NC:handlerProp RDF:resource="urn:mimetype:handler:application/pdf"/>
</RDF:Description>
I have disabled all Add-ons to see if any of them was causing this, but no luck. I suspect it's either some add-on or OS app association (like OS X Preview) that's causing this, but I can't find the culprit.
Has anybody ever encountered this? It's very straightforward to fix, but I would like to understand exactly what is going on because it sounds somewhat wrong to send "image/ipeg" as the Media Type for PDF.

Answer 1

One way that odd content types can be added to the mimeTypes.rdf file is when you are presented with the download dialog (Open/Save/Cancel) for a novel content type and you associate it with a known application. But tracking down the exact site that caused this (assuming it was a misconfigured server) could be impossible.
On Windows, bizarre things also could be incorporated from the Windows registry, but I don't know whether there is anything equivalent on Mac.