Thanks for the great plugin. It works dandy for me on DreamHost shared hosting for normal uploads through the Dashboard Media Library.
Would it be possible to support transferring files that are uploaded by plugins by other means? For example, I'm using the WooCommerce Product Add-Ons Extension, which allows site visitors to upload files. It uses the wp_handle_upload function for this. Files uploaded this way neither get transferred to S3 nor get their URLs replaced (or I'm doing something wrong). Is there some way I can hook into the S3 plugin to make this happen, and/or would it be possible to add this as a feature in future versions?