"Can't dump file" error occurs when using Paperclip attachments with Multi-step form in Rails 3

Question

I'm having the same issue mentioned here: Multi-step form in Rails 3 with Paperclip attachments

"TypeError in..." "can't dump File"

However, I can't seem to wrap my head around the solution mentioned in the answer:

unless @model.valid?
  @model.image.clear
  @model.image.queued_for_write.clear
end

Where do I put the above lines?

Here's my code for my controller:

  def new
    session[:member_params] ||= {}
    @member = Member.new(session[:member_params])
    @member.current_step = session[:member_step]
  end

  def create
    session[:member_params].deep_merge!(params[:member]) if params[:member]
    @member = Member.new(session[:member_params])

    @member.current_step = session[:member_step]
    if @member.valid?
      if params[:back_button]
        @member.previous_step
      elsif @member.last_step?
        @member.save if @member.all_valid?
      else
        @member.next_step
      end
      session[:member_step] = @member.current_step
    end

    if @member.new_record?
      render "new"
    else
      session[:member_step] = session[:member_params] = nil
      flash[:notice] = "Personal Data Sheet saved!"
      redirect_to @member
    end
  end

I tried putting it here after the declaration of @member, but no good:

session[:member_params].deep_merge!(params[:member]) if params[:member]
@member = Member.new(session[:member_params])

unless @member.valid?
  @member.image.clear
  @member.image.queued_for_write.clear
end

P.S. I thought of appending my question to the question here: Multi-step form in Rails 3 with Paperclip attachments But I'm not sure if it's okay to post-follow-up questions on the same issue/question.