You can use post requests to send files to your remote server. You backend code should read request data and then store the file onto the disk or any object storage like s3. Most backend web frameworks have many libraries to store files received in an HTTP request directly to s3 Most of the web development frameworks have the capability to guess the mimetype but here since you know it's video/mp4 you can just save it.

I must warn you if you are trying to upload huge files it might be a better idea to use chunked uploads. This gives you ability to pause and resume and is robust to network failures.