The automatically created forms don't understand how to handle file uploads.

[opencoca]

Forms should use enctype="multipart/form-data" if files are in the model.

Alternatively, forms could always use enctype="multipart/form-data" and developers could comment it out if they needed increased efficiency.

Example of form with working file uploads:
<form enctype="multipart/form-data" method="post" action="/foo/">