Fix after FilePreviews API breaking changes

[jpadilla]

Docs: http://filepreviews.io/docs/
Example libraries already updated:

• https://github.com/getblimp/filepreviews-python
• https://github.com/getblimp/filepreviews-node

[elbuo8]

@jpadilla have there been more changes?

[jpadilla]

Not yet