If you hadn't purchased MySQLi Server Behaviors the price would have been $299.99.... the idea was that we didn't want to penalize people for buying early, so you get a discount based on that purchase.
The code you have seems like it would work. It is custom code, not using our extension, but it looks very similar to what our extension uses in terms of chunking the download and buffering.
Is the value of the $fsize variable correct, or is it the 1MB size as well? Have you tried with other browsers? It might not be an issue with your code at all.