This bug is reproducible only in dev environment.

Steps to reproduce:

1. Create Landing Page
2. Add Form Block
   1. Add Text Line field
   2. Add File field
   3. Add notification e-mail address
3. Publish Landing Page
4. Go to the front-end and fill the form
5. Submit form

Nothing will happen, but in the Developer Tools you can see 500 error and response:

{"valid":false,"invalidFields":[]}

This bug appears only in dev environment, because Exception comes from Twig template:
https://github.com/ezsystems/ezstudio-form-builder/blob/master/bundle/Resources/views/email/send_email.html.twig#L19

There is a Array to string conversion... warning.