Unable to attach files to email using code from docs

[RandomJo]

Expected behaviour

I expect to be be able to attach uploaded files to an email as I could with previous versions of wagtailstreamforms.

Actual behaviour

After upgrading to wagtailstreamforms==4.0.4, I get the following error when using the code in the docs to attach to an email. The code in the docs was working in wagtailstreamforms==3.21

Traceback (most recent call last):
  File "[PATH_TO_ENV]/lib/python3.8/site-packages/django/core/handlers/exception.py", line 56, in inner
    response = get_response(request)
  File "[PATH_TO_ENV]/lib/python3.8/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "[PATH_TO_ENV]/lib/python3.8/site-packages/wagtail/views.py", line 24, in serve
    result = fn(page, request, args, kwargs)
  File "[PATH_TO_PROJECT]/custom_forms/wagtailstreamforms_hooks.py", line 46, in process_form
    form_def.process_form_submission(form)
  File "[PATH_TO_ENV]/lib/python3.8/site-packages/wagtailstreamforms/models/form.py", line 184, in process_form_submission
    fn(self, form)
  File "[PATH_TO_PROJECT]//custom_forms/wagtailstreamforms_hooks.py", line 137, in email_submission
    file.open()
  File "[PATH_TO_ENV]/lib/python3.8/site-packages/django/core/files/uploadedfile.py", line 115, in open
    self.file.seek(0)
ValueError: I/O operation on closed file.

Here's my code:

 # create the email message
    email = EmailMessage(
        subject=subject,
        body=content,
        from_email=from_address,
        to=addresses,
    )

    # attach any files submitted
    for field in form.files:
        print(form.files.getlist(field))
        for file in form.files.getlist(field):
            print(file)
            print(file.__dict__)
            file.seek(0)
            email.attach(file.name, file.read(), file.content_type)

    # finally send the email
    email.send(fail_silently=True)

Steps to reproduce the behaviour

1. Create a stream form with a file upload field (single or multiple).
2. Add the form to a page.
3. View the form on the site.
4. Upload a file (any file type) and submit.
5. You should see the error.