Cannot explicitly set format on a single file.

**- Do you want to request a *feature* or report a *bug*?**
bug

**- What is the current behavior?**
If I am using a file-type collection:
```yml
- label: "Pages"
name: "pages"
files:
- label: "About Page"
name: "about"
file: "site/content/about.yml"
fields:
- {label: Title, name: title, widget: string}
- {label: Intro, name: intro, widget: markdown}
```
I can set a `format` on the top-level collection (Pages), but not separately on each file (About Page).
This seems to have started in v1.0.

**- If the current behavior is a bug, please provide the steps to reproduce.**

**- What is the expected behavior?**
I should be able to set an explicit `format` separately on each file in a file-type collection.

**- Please mention your CMS, node.js, and operating system version.**

v1.0.3