The best way is to upload it to another service that specializes in serving videos, like Loops, Pixelfed, etc. and posting a link here. Lemmy servers are volunteer-ran, and streaming content is a rather heavy load in comparison to the mostly textual handling it does.

It depends on the configuration of the instance. Most larger instances disallow it due to the storage requirements of video files.

Are using an app to access your instance ?

On voyager you have an icon to upload files, directly on browser I’m not sure