read-from-stream-to-file.cs

var tempFilename = Path.Combine(Path.GetTempPath(), $"{Guid.NewGuid()}.tmp");
using (var stream = new FileStream(tempFilename, FileMode.CreateNew))
{
    const int chunkSize = 1024;
    var buffer = new byte[chunkSize];
    var bytesRead = 0;
    do
    {
        bytesRead = await fileSection.FileStream.ReadAsync(buffer, 0, buffer.Length);
        await stream.WriteAsync(buffer, 0, bytesRead);
    } while (bytesRead > 0);
}