Describe the bug
When a .pdf file is uploaded to the Media Gallery, it's preview card is blank. No thumbnail is generated for it. Note that this remains true after a page refresh. Thumbnails are generated for images.

To Reproduce

Expected behavior
Either a Thumbnail, or a PDF icon is displayed on the file's preview card.

Observations

• This is a different issue to Media Gallery Thumbnail For Image Missing Until Refresh (Gitlab Self-hosted Repo) #1985 as no image is displayed even after a page refresh.
• If a preview cannot be generated for PDFs, an icon should be displayed as current behaviour makes it look like upload failed.

Screenshots

Applicable Versions:

• Netlify CMS version: netlify-cms 2.3.2
• Git provider: Gitlab
• OS: OSX
• Browser version: 71.0.3578.98
• Node.JS version: 8.11.3