I cannot reproduce this on Elgg 4 and Elgg 5 both. Everything works well for me without any 3rd party plugins ('File' plugin is activated).

The video format may not be supported. Try another mp4 video file.

You're absolutely right. I suppose it means I need to look at the server and ffmpeg I would guess to support most if not all file formats. I'm glad it wasn't elgg not having media support right out of the box.

Thank you.

One quick question. When I edit a video and upload a new file, does it replace the previous one or does it leave it behind? If it leaves it behind, how does that old stuff get cleaned out?

I suppose it means I need to look at the server and ffmpeg I would guess to support most if not all file formats

I don't think you need ffmpeg and additional software.

The video should play automatically through the HTML5 API.
But there may be problems similar to yours.
Mostly problems arise with the MOV video format.

To avoid this, additional plugins and software (e.g., ffmpeg) are needed.
For example, this plugin converts any media file, allowing them to be played on a website page.

Here's what it looks like.

When I edit a video and upload a new file, does it replace the previous one or does it leave it behind?

The old file will be replaced by the new one.

If it leaves it behind, how does that old stuff get cleaned out?

The old file is deleted and the new one is stored in the data folder.

Thank you, that certainly explains everything.

If this is the right place to keep asking, I have more questions. Maybe it could help someone else who finds this.

About media files, is there a way to show a preview?
Testing in group, files show no previews.
Testing almost all of the widgets in a profile shows no widgets.
I see that I can paste an image and see a preview in comments but no obvious options for users to know this.
No option to paste image in message board.
I notice I cannot upload an image to show what I mean in this post, I can only share a url.

I assume there is a way to put all my activities into the center, like a feed but I'm not seeing this yet. I did come across something called 'river' so I'll research more but it looked like lots of coding and I'm not a dev.

Maybe it's because I'm using the default theme at the moment.
However, looking at the plugins section, there are only two themes updated to 5.x. I see a ton of old plugins and themes, not updated for a very long time.

What does it mean? Does it mean that most are coding their own or that the plugin community is not that active? I also read that themes aren't really needed anymore, just customize as needed but again, I'm not a dev so everything I would need, I'd have to pay for.

Are there any paid themes and plugins that can help ensure they will continue to be updated?

Lot's to play with, lots to learn.

Currently, there're no Elgg 5 plugins for previewing uploaded files. There're some plugins but for Elgg 4 only.

You may want to see at these plugins also:
https://elgg.org/plugins/3288617
https://elgg.org/plugins/2932901
https://elgg.org/plugins/3284971

In this Telegram channel we keep track of all new free and paid plugins for Elgg.

Our team has a marketplace. 
Since Elgg 5 release was released recently, we don't have products for this version yet, but we're working on updates.