In this specific case there requirement of "file_upload" is not a plugin by that name. It's the Files plugin bundles with Elgg that provides the functionality requested by the Embed plugin with "file_upload". Just activate the Files plugin and then you should be able to also activate the Embed plugin.

Uploaded videos can't be viewed directly from the site without some additional plugins and server setup. First of all, not all video formats can't be played directly within a browser. So, the videos need to be converted in a format that can't be played either by a server-side video player or can be played by a html5 compatible format that the browser in use understand. In any case you most likely need the ffmpeg package installed on the server (not supported necessarily by all hosting plans). ffmpeg will provide the video conversion capability. Then you also need an Elgg plugin that deals with the conversion process and then provides the videos in a form the user can play them in the browser.

The File Viewer plugin (https://elgg.org/plugins/2648090) is one possibility (requires some other plugins additionally) or the iZAP Videos plugin (https://elgg.org/plugins/1913474) that also allows embedding of offserver videos for example from Youtube. In any case, you will require ffmpeg to be installed on the server in both cases.