dossiers: allow auto upload of attachments

app/controllers/champs/piece_justificative_controller.rb

@@ -0,0 +1,21 @@
+class Champs::PieceJustificativeController < ApplicationController
+  before_action :authenticate_logged_user!
+
+  def update
+    @champ = policy_scope(Champ).find(params[:champ_id])
+
+    @champ.piece_justificative_file.attach(params[:blob_signed_id])
+    if @champ.save
+      render :show
+    else
+      errors = @champ.errors.full_messages
+
+      # Before Rails 6, the attachment was persisted to database
+      # by 'attach', even before calling save.
+      # So until we're on Rails 6, we need to purge the file explicitely.
+      @champ.piece_justificative_file.purge_later
+
+      render :json => { errors: errors }, :status => 422
+    end
+  end
+end