As the help topic notes, the page continues to upload the file even though the confirmation dialog is showing. So I have to assume your uploading a file that doesn't take very long to upload, but is above 4MB in size, so the max file size is reached and the error appears.
I would re-order your code and just cancel the request and then display an alert that the file size has been reached. Either that, or increase the maxRequestLength in your web.config and see if that helps.
The confirmation dialog popup never shows. We recieve a filesize exceeded error right away.
answered on 26 Sep 2011, 03:09 PM
Please consult with the following help article showing how to configure for large file uploads in IIS7+
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