I have no experience with Plupload. You can try uploadify or swfupload , Both allow you to upload multiple files and any important Plupload and require the installation of Flash. In my opinion, adding is easier to implement than swfupload, and it is very customizable.
Here are some examples:
You can find more examples on github.com. And you need a plugin to attach files to the back-end, for example paperclip or attachment-fu.
You can find a few tips when previewing the images before submitting the form here . And if you use nested forms, here is complex-form-examples . See the unobtrusive jquery branch . You can add a new child in the onComplete uploadify event.
I use uploadify with paperclip, preview and nested forms.
UPDATED.
Here are some examples of using Plupload in RoR:
Download Files Using Rails 3, Plupload, and CarrierWave
Ruby on Rails + Paperclip + PlUpload
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