PDF viewer does not play movies

Summary

When loading a PDF which features animated content, such as Flash, H.264 video, or GIFs, the animated content is not animated.

Steps to reproduce

1. Upload a PDF featuring animated content.
2. Attempt to play the animated content.

Example Project

https://gitlab.com/slondr/pdf-example

What is the current bug behavior?

A "play" button appears overlayed on the animated content, but the content does not play when the button is clicked.

What is the expected correct behavior?

No play button should appear; instead, the animated content should play as it does in Okular, Adobe Acrobat, etc.

Output of checks

This bug happens on GitLab.com