Allow file uploads in snippet forms

I had a model registered as a snippet in Wagtail where one of the fields was something like this:

```python
file = models.FileField(upload_to='uploads')
```

When I tried to upload a file within Wagtail's admin interface, the file never got stored and associated with the model. I fixed the issue by changing the HTML templates of the forms to post their data as enctype="multipart/form-data".