Improve helper

With this update UploaderHelper will be able to retrieve file path both from mapping name and from field name. It will go for mapping name first and the fallback to field name. 

This is useful for 2 reasons:

1 - we can keep compatibility with users that uses  0.9.0 because the helper will continue to work
2 - in certain condition I'm using the helper with different object and different mappings but with the same mapped field. With this new implementation I can avoid differentiate my template based on mapping name (which is good as I can infer the mapping name from field metadata).

I'd like some :+1: before merging this, what do you think @K-Phoen ?