Added file uploads

[thomaskcr]

When an AWS access key and bucket are configured, files can be uploaded by dropping them onto a page. Information about the attached files are stored in the front matter of the page so pages with uploads are still normal markdown.

The markdown for the page in the attached image:

---
attachments:
- filename: data_export-out_v4.csv
  handler: S3
  key: wiki/files/201807120616_F8sZrW/data_export-out_v4.csv
---


# Test Page

This is a test example page.