Video processing strategy.

### Your use case

#### What would you like to do?
1. Play while downloading;
2. Clarify the condition that a video can be previewed

#### Why would you like to do it?
There are problems with the current version of the processing of video:
1. If the video is not completed, it will not start playing. This will lead to longer waiting time.
2. I have recently done an experiment: Test Element's response to some video files. The encoding and compression rates of these files are the same, but the duration and file size are different.
The final result reaction is different:
Some can play (show preview images in the conversation), and some cannot (displayed in the form of files in the conversation).
However, I did not find the rules. At first I thought it was a video that exceeded the specified size and would not be played (for example, 300 MB), but as the uploaded documents became more and more, I found that it was not.

#### How would you like to achieve it?
n/a

### Have you considered any alternatives?

Send the link to watch the video at the designated site to replace the built -in video playback function of Element.

### Additional context

n/a

### Are you willing to provide a PR?

No