New EssenceFile content option `only` and `except`

You can now pass only and/or except option to EssenceFile content settings. It will not/only load attachments with matching content type.

Example:

contents:
- name: pdf
  type: EssenceFile
  settings:
    only: pdf

Will only show pdfs in the attachment library overlay. Using `except` will do the opposite.
Merge pull request #682 from magiclabs/feature/assign_attachment_only

New EssenceFile content option `only` and `except`. Closes #626