I am looking to overwrite the existing upload functionality to save the file on a remote storage service. The remote storage has an HTTP interface that allow me to post file and return an addressable URL back. The reason for doing this is that the remote storage service has large amount of space and is automatically replicated for high-availability.
I have already figured out how to send files across. However, I am not sure which hook/function I should use to overwrite the existing behavior.