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TL;DR: No data is sent to the server, the images resizing is done by the browser of the person that is uploading the image, then the minimized image is uploaded to the WordPress server.
It works by replacing those WordPress scripts:
- /wp-includes/js/plupload/plupload.js?ver=2.1.9 – replaces with either [plugin_dir]/libs/plupload-2.1.9/js/plupload.min.js?ver=2.1.9 (default) or [plugin_dir]/libs/plupload-2.3.9/js/plupload.min.js?ver=2.3.9
- /wp-includes/js/plupload/moxie.js?ver=1.3.5 – replaces with either [plugin_dir]/libs/plupload-2.1.9/js/moxie.min.js?ver=1.3.5 (default) or [plugin_dir]/libs/plupload-2.3.9/js/moxie.min.js?ver=1.5.7
The scripts shipped with the plugin are:
The client-side resize is helpful in that way that it offloads the images resize processing from the server to the clients that are uploading those images.
The plugin has only a few settings at the moment, which can be seen under “Media” -> “Resize before upload”.