Safari downloads as .dms files. #581

For Bug Reports

• BookStack Version: BookStack v0.18.4
• PHP Version: PHP 7.0.22-0ubuntu0.16.04.1
• MySQL Version: mysql Ver 14.14 Distrib 5.7.20, for Linux (x86_64) using EditLine wrapper

Expected Behavior

Exports should be downloaded with the correct file extension (.pdf, .html, .txt)

Current Behavior

Exports download as page.dms, html.dms, or plaintext.dms when using Safari Version 11.0 (12604.1.38.1.7). If I change the extension after the file is downloaded, it opens correctly.

It also exports correctly using Chrome.

Steps to Reproduce

Install using the Ubuntu 16.04 script.
Create a new book/page.
Click any of the export links.