I tried to save uploaded images but my log send me :
Unpermitted parameter: design_files
However this is the hash parameters my controller is sending back to me :
"design_files"=>[#<ActionDispatch::Http::UploadedFile:0x007fce350cbcb8 @tempfile=#<Tempfile:/var/folders/0w/pznf2h6j0q9gqcbcthg1kgm40000gn/T/RackMultipart20170315-5734-26bwb.png>, @original_filename="ebf84ecaf15f684b53f18b9313cd1325.png", @content_type="image/png", @headers="Content-Disposition: form-data; name=\"cover[design_files][]\"; filename=\"ebf84ecaf15f684b53f18b9313cd1325.png\"\r\nContent-Type: image/png\r\n">]
So the params design_files does exist, but the results is an array, may be the problem come from there.
I let you see the rest of my code :
covers_controller.rb
class CoversController < ApplicationController
  before_action :set_cover, only: [:show, :edit, :update, :destroy]
  def new
    @book = set_book
    @cover = @book.build_cover
  end
  def create
    @book = set_book
    @cover = @book.build_cover(cover_params)
    if @cover.save!
      redirect_to new_command_path(commandable_id: @cover, commandable_type: @cover.class)
    else
      render :new
    end
  end
  ....
   private
  ...
  def cover_params
    params.require(:cover).permit(:design_files) 
                                   # according to other solutions find on stack I tried : design_files_attributes: {:design_files}, also tried {design_files: :preview}                                
  end
end
cover.rb
class Cover < ApplicationRecord
  validates_presence_of :design_files
  has_attached_file :design_files
  do_not_validate_attachment_file_type :design_files
end
covers/_form
= form_for cover, html: { multipart: true } do |f|
  - if cover.errors.any?
    #error_explanation
      h2
        = pluralize(cover.errors.count, "error")
        |  prohibited this cover from being saved:
      ul
        - cover.errors.full_messages.each do |message|
          li
            = message
  = f.label :design_files
  = f.file_field :design_files, multiple: true
  .actions
    = f.submit
 
     
    