ReadAsync(Stream, Int64, Int64)
Writes the content of the file to the specified stream.
By default ASP.NET buffers content on server side before sending output. You must turn off buffering to
eliminate keeping entire file content in memory before sending:
HttpContext.Current.Response.BufferOutput = false;
Client application can request only a part of a file specifying Range header.
Download managers may use this header to download single file using several threads at a time.
public Task ReadAsync(Stream output, long startIndex, long count)