If you need to stream a video from webapi, you can use the following codes. The code is taken from stackvoerflow. I did not write it.
If you're using .net core, use the following code.
There are loads of sample in the internet, but i think this works well for .net 4.6.
You probably need to know that if you not sure exact size of your files, try PushStreamContent.
Otherwise, you can just use StreamContent.