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GitHub User: pbevin
I found that in Rails 4, setting name: in f.file_field didn't work: Rails appended  to the end of the field name regardless. I had to replace this:
<%= f.file_field :image, multiple: true, name: "painting[image]" %>
with plain HTML, like this:
<input id="painting_image" multiple="multiple" name="painting[image]" type="file" />