There are quite a few ways to cause this error. Some of them are listed here:
Here was my solution:
If you have multiple forms on a single page then make sure *EVERY SINGLE ONE* has
<form method="POST" enctype="multipart/form-data" action="whatever">
Even if the form containing your
<input type="file" name="the_name"/>
tag already has the correct enctype, Mozilla Firefox 3.6.x will give an error in the Tools->Error Console about refusing to upload the file due to incorrect enctype.