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I don’t think your conditions variable is supposed to contain a ‘fileExt’ key-value pair. fileExt is a property of Uploadify, not Amazon S3 POST process, and options is how you configure the Uploadify widget.
The conditions variable is what gets serialized into the policy file that gets sent to Amazon S3 and explains the rules file uploads must follow.
If you remove ‘fileExt’ from conditions, that should fix it.
Source:stackexchange.com