Avoid Mongoid warning about overwriting Asset field
Avoid Mongoid warning about overwriting Asset field

We were seeing the following warning in the Rails log:

    Overwriting existing field file in class Asset

This was happening because the uploader declaration was also declaring
the `file` field [1]:

    mount_uploader :file, AssetUploader

The latter set the field type to be `Object` whereas the former set the
field type to `String`. However the latter took precedence anyway and so
removing the former has no net effect.

Fixes #69.

[1]: https://github.com/carrierwaveuploader/carrierwave-mongoid/blob/v0.8.1/lib/carrierwave/mongoid.rb#L15

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager