GitHub User: wyaeld
The SVN blog discusses that. The short answer is you can't use conditionals that can change when the template itself doesn't. Impacts system design a bit.
There is a missing step here, and in the example code.
When uploading images to S3, you want to specify the Content-Type, otherwise S3 will just default it to "application/octet-stream", which can cause your browser to throw warnings when you retrieve the file.
This caught me out, because Content-Type is listed in the multi-part form request as a component of the "file" input, however crucially S3 will NOT look at that. You need to pass it again.
In my case, using jqueryFileUpload as Ryan did, I have
<input id="content-type" name="Content-Type" type="hidden" value="" />