Invalid content type

I have a form that allows the user to upload a photo.
It works fine.
However, the CF logs are showing repeated errors on the processing page, like so:

Invalid content type: ""application/x-www-form-urlencoded"".<br>CFFILE action=""upload"" requires forms to use enctype=""multipart/form-data""

I added a try/catch around the cffile tag to try and generate more information about the browser, submitting page, etc., but have yet to determine why this error is being generated.

Does anyone know of a browser that would modify the enctype attribute?
Could someone be attempting to upload a file NOT of type JPG, so that when the cffile tag runs it geneates this error?

Any insight would be greatly appreciated.
Thanks,
bh