Colorization of the link to uploaded documents #1189

Describe the bug
The link to the document downloaded in the article is indicated in red as if the link was pointing to an empty page .

To Reproduce
Steps to reproduce the behavior:

1. Upload a Pdf file or other extension
2. Insert link to this this document in article
3. Save the page
4. See error

Expected behavior
The link must be colorized in blue

Screenshots

Host Info:

• OS: Ubuntu 18.04.3
• Wiki.js version: 2.0.0-rc.17
• Database engine: MariaDB 10.3.18