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I'm running the combination of Media 7.x-2.x-u6 + Multiforms + PLUpload that seems to function, except that PLUpload isn't grabbing the correct set of allowed file extensions from the $element variable. This might be due to changes in the Media module or how Multiforms makes the PLUpload widget available -- I'm not well versed enough in the Drupal API to figure it out. But it is clearly defaulting to the values stored at the end of plupload.module.