File API Upload - XMLHttpRequest onload event not firing on larger files

I'm using the new File API for uploading files to my site. I have everything working as expected. Files are submitted and uploaded successfully.

The only problem I'm finding is that my onload event for the XMLHttpRequest does not fire on files with a larger file size. Any ideas on why this would work most of the time but fail for larger files?

I've tried sticking code in both the XMLHttpRequest and the XMLHttpRequest.upload onload events. Both seem to fail with large files. I'm trying to stay away from the workaround of sniffing for 100% in th开发者_开发知识库e progress event to denote completion.