If time permits, I will work on the this idea during GSoC (or even after GSoC), after completing my proposed project: Rich Text Editor Integration.
Idea: Attachment Improvements
So far, I have succeeded in integrating the Plupload into the phpBB core files (posting_attach_body.html). I have set the allowed file types, the maximum file size, resized images and enabled ‘select multiple files’ option.
Next step in the project is to learn how phpBB upload script works and integrate Plupload with the upload.php. Also attached file comment seems to be an issue.