Describe the bug
The media view shouldn't display dotfiles.

To Reproduce

1. Add a dotfile to the configured media_folder
2. The dotfile shows up in the media view.

Expected behavior
Dotfiles should be ignored.

Screenshots
https://take.ms/28TGh

Applicable Versions:

• Netlify CMS version: 2.9.1
• Git provider: GitHub
• OS: macOS 10.14.5
• Browser version Chrome 74

CMS configuration
https://github.com/uplinkhq/website/blob/master/source/admin/config.yml