Fix missing enctype="multipart/form-data" when needed

When uploading a file through djadmin2, the file is left out
of the form, as the form encoding type is not set to
multipart/form-data
this results in false form errors (this field is required)
as the file isn't included in the request object.

This is a fix for this.

- add app `example.files` with tests, based on example2.
  for completness.
- add check in base update template which checks if
  form.is_multitype and adds the encoding for it
- update gitignore to ignore the uploaded media folder
- add test to blog to ensure that the create form is not
  multipart encoded (as it has no files)