TYPO3 Core

This feature was included in version 7.5.: https://review.typo3.org/#/c/41608/
In version 7.6 it doesn't work or it is missing again!???

What does that mean "Candidate for Major Version"? Version 8?

@Johannes: It is supported by the old mediace extension, which was extraced from the Core. You may still use this extension of course.

In general we need a solution for assigning a preview image for any kind of file, where the system is not capable of generating one.
In addition to that we need some generic mechanics how those preview images are created. While a simple file upload of a prerendered image from the desktop is sufficient for many file types, it might be handy to offer a preview image selector for videos based on a frame number (leveraging some video library like ffmpeg for that).

In the meantime, I have created an extension that might be interesting for you? https://packagist.org/packages/b13/poster-image

For YouTube and Vimeo I suggest to add the posterimage there.