_form.html.erb

<%= form_for @analysis, :html => { :multipart => true }  do |f| %>
    <%= render "shared/errors", object: @analysis %>
    <fieldset>
      <ol>
        <li>
          <%= f.label :title %>
          <%= f.text_field :title, size: 50, autofocus: true %>
        </li>
        <li>
          <%= f.label :body %>
          <%= f.text_area :body, rows: 20, cols: 100 %>
        </li>
         <li>
          <%= f.label :trend %>
          <%= f.check_box :trend %>
        </li>
        <li>
          <%= f.label :images %>
          <%= f.file_field :images, multiple: true %>
          <%= f.hidden_field :images_cache, multiple: true %>
        </li>
      </ol>
      <div id="submit">
        <% if @analysis.new_record? %>
            <%= f.submit 'Create Analysis', class: 'button' %>
            <%= link_to 'Cancel', :back , class: 'button' %>
        <% else %>
            <%= f.submit 'Update Analysis', class: 'button' %>
            <%= link_to 'Cancel', @analysis, class: 'button' %>
        <% end %>

      </div>
    </fieldset>
<% end %>

analysis.rb

class Analysis < ActiveRecord::Base
  mount_uploader :images, ImagesUploader

  validates :title, presence: true
  validates :body, presence: true
end

analysis_controller.rb

class AnalysesController < ApplicationController

  def index
    @analyses = Analysis.all
  end

  def new
    @analysis = Analysis.new
  end

  def create
    @analysis = Analysis.new(analysis_params)
    if @analysis.save
      flash[:notice] = 'Analysis successfully created.'
      redirect_to @analysis
    else
      render :new
    end
  end

  def edit
    @analysis = Analysis.find(params[:id])
  end

  def update
    @analysis = Analysis.find(params[:id])
    # fail
    if @analysis.update(analysis_params)
      redirect_to @analysis, notice: 'Analysis successfully updated!'
    else
      render :edit
    end
  end

  def show
    @analysis = Analysis.find(params[:id])
  end

  def analysis_params
    params.require(:analysis).permit(:title, :body, :trend, { images: [] } )
  end
end

console result

Processing by AnalysesController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"T3GT2UXd7kWkvou5+eVq8UjBepKIqs8wSsQDcOyF5TTf4wyVhR3g7Np3wSTnZv+A4jfy1PDoMnNu32qMvs4UxQ==", "analysis"=>{"title"=>"Analysis Number One", "body"=>"Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet, consetetur sadipscing elitr, sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat, sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren, no sea takimata sanctus est Lorem ipsum dolor sit amet.   \r\n", "trend"=>"1", "images"=>[#<ActionDispatch::Http::UploadedFile:0x007fabb2ba6390 @tempfile=#<Tempfile:/tmp/RackMultipart20160106-17421-wor4pn.png>, @original_filename="2015-07-12_TF.png", @content_type="image/png", @headers="Content-Disposition: form-data; name=\"analysis[images][]\"; filename=\"2015-07-12_TF.png\"\r\nContent-Type: image/png\r\n">]}, "commit"=>"Update Analysis", "id"=>"1"}
  Analysis Load (0.1ms)  SELECT  `analyses`.* FROM `analyses` WHERE `analyses`.`id` = 1 LIMIT 1
   (0.1ms)  BEGIN
   (0.2ms)  ROLLBACK
Completed 500 Internal Server Error in 5ms (ActiveRecord: 0.3ms)

TypeError (no implicit conversion of nil into String):
  app/controllers/analyses_controller.rb:28:in `update'


  Rendered /home/andreas/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.5/lib/action_dispatch/middleware/templates/rescues/_source.erb (13.1ms)
  Rendered /home/andreas/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.5/lib/action_dispatch/middleware/templates/rescues/_trace.html.erb (1.9ms)
  Rendered /home/andreas/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.5/lib/action_dispatch/middleware/templates/rescues/_request_and_response.html.erb (0.8ms)
  Rendered /home/andreas/.rvm/gems/ruby-2.1.5/gems/actionpack-4.2.5/lib/action_dispatch/middleware/templates/rescues/diagnostics.html.erb within rescues/layout (24.3ms)

error

TypeError in AnalysesController#create
no implicit conversion of nil into String

Extracted source (around line #12):
10
11
12
13
14
15
              

  def create
    @analysis = Analysis.new(analysis_params)
    if @analysis.save
      flash[:notice] = 'Analysis successfully created.'
      redirect_to @analysis

Rails.root: /home/andreas/rails/alphatrade

Application Trace | Framework Trace | Full Trace
app/controllers/analyses_controller.rb:12:in `create'
Request

Parameters:

{"utf8"=>"✓",
 "authenticity_token"=>"o3RhwSX3A0qT1pOjIVmVxVI2/ZOCNkjjBvUz6YFJjRgz5v6N5TcN4+0f2T4/2gC0+MB11fp0taAi7loV0wJ86Q==",
 "analysis"=>{"title"=>"Analysis Number One",
 "body"=>"Lorem ipsum dolor sit amet,
 consetetur sadipscing elitr,
 sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat,
 sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
 no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet,
 consetetur sadipscing elitr,
 sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat,
 sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
 no sea takimata sanctus est Lorem ipsum dolor sit amet. Lorem ipsum dolor sit amet,
 consetetur sadipscing elitr,
 sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat,
 sed diam voluptua. At vero eos et accusam et justo duo dolores et ea rebum. Stet clita kasd gubergren,
 no sea takimata sanctus est Lorem ipsum dolor sit amet.   \r\n",
 "trend"=>"1",
 "images"=>[#<ActionDispatch::Http::UploadedFile:0x007fabb28a8d50 @tempfile=#<Tempfile:/tmp/RackMultipart20160106-17421-1o0zq4a.png>,
 @original_filename="2015-07-12_ES.png",
 @content_type="image/png",
 @headers="Content-Disposition: form-data; name=\"analysis[images][]\"; filename=\"2015-07-12_ES.png\"\r\nContent-Type: image/png\r\n">,
 #<ActionDispatch::Http::UploadedFile:0x007fabb28a8d28 @tempfile=#<Tempfile:/tmp/RackMultipart20160106-17421-1t97ixv.png>,
 @original_filename="2015-07-12_NQ.png",
 @content_type="image/png",
 @headers="Content-Disposition: form-data; name=\"analysis[images][]\"; filename=\"2015-07-12_NQ.png\"\r\nContent-Type: image/png\r\n">],
 "images_cache"=>[""]},
 "commit"=>"Create Analysis"}

image_uploader.rb

class ImagesUploader < CarrierWave::Uploader::Base
  include CarrierWave::RMagick

  storage :file

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  version :thumb do
    process resize_to_fit: [150, 150]
  end

  def extension_whitelist
    %w(jpg jpeg gif png)
  end
end

migration.rb

class AddImagesToAnalysis < ActiveRecord::Migration
  def change
    add_column :analyses, :images, :text
  end
end