If your PHP (or any other server side code) is receiving a single $_POST field of raw data, looking more or less like this, instead of receiving your form fields nicely:
Content-Disposition: form-data; name=”form-file-1″; filename=”my document.doc”
Content-Disposition: form-data; name=”form-field-1″
(that WebKitFormBoundary is probably the sign that you are using Chrome)
or really anything else which is not this:
In my case, I was getting this problem because I was using the js FormData object to extract form data and then sending it the same way it used to do with “regular” form data. Turns out the content-type header should be set differently. Maybe I saved you a couple of hours of searching.