How can I upload files in Rails

I would like to know how to generate a form in Rails so that I can upload files without problems, I have read several tutorials, but it still costs me a bit to do it.

Controller:

class ContratosController < ApplicationController
  DOCUMENTO_TOKEN = "public/uploads/documentotoken"

  def index
    @tituloindex = "Emitir Asset";
    @tituloinfo = "Info de Contrato";
    @coins = params[:coins];

    if request.post?
      documento = params[:documento];

      #Nombre Original del Documento
      nombre = documento.original_filename;

      #Directorio donde se va a guardar
      directorio = DOCUMENTO_TOKEN;

      #Extension del archivo
      extension = nombre.slice(nombre.rindex("."), nombre.length).downcase;

      if extension == ".pdf" or extension == ".jpg"
        #ruta del archivo
        path = File.join(directorio, nombre);
      else
        @formato_erroneo = true;
      end
    end
  end

  def info
  end
end

View:

<h1><%= @tituloindex; %></h1>
<table width="600px">
<%= form_tag({:controller => "contratos", :action => "index"}, :method => "post", :multipart => true) do %>
  <tr>
    <td><%= label_tag "coins", "Initial Coins" %></td>
    <td><%= text_field_tag "coins", "", :size => 20, :maxlenght => 5, :placeholder => "coins", :required => "required" %></td>
  </tr>
  <tr>
    <td><%= label_tag "decimals", "decimals" %></td>
    <td><%= text_field_tag "decimals", "", :size => 20, :maxlenght => 5, :placeholder => "decimals", :required => "required" %></td>
  </tr>
  <tr>
    <td><%= label_tag "tokenname", "Token Name" %></td>
    <td><%= text_field_tag "tokenname", "", :size => 20, :maxlenght => 5, :placeholder => "Token Name", :required => "required" %></td>
  </tr>
  <tr>
    <td><%= label_tag "tokenpreffix", "Token Preffix" %></td>
    <td><%= text_field_tag "tokenpreffix", "", :size => 20, :maxlenght => 5, :placeholder => "Token Preffix", :required => "required" %></td>
  </tr>
  <tr>
    <td><%= label_tag "selecttoken", "Select Token" %></td>
    <td><%= select_tag "selecttoken",
                       "<option>crowdsale</option>
                        <option>ShareHolder Company</option>
                        <option>Simple Token</option>
                        <option>Pannic</option>
                        <option>comoditie</option>".html_safe %></td>
  </tr>
  <tr>
    <td><%=label_tag "documento","Documento de Identidad (Solo *.pdf o *.jpg)"%></td>
    <td><%= file_field_tag "documento", :required => "required" %></td>
  </tr>
</table>
<%= submit_tag "Enviar" %>
<% end %>

<div id="resultados" >
  <% if @coins %>
    <%= "Se han enviado #{@coins} Lescovex ha su cuenta" %>
  <% else %>
    <%= "verifique bien los datos" %>
  <% end %>
</div>