No route matches [POST] "/" Rails Form_tag

The error you show is because you are using root_pathfor your form submit, but you should use posts_pathfor your form to submit to createyour controller action PostsController; example:

<%= form_tag(posts_path, multipart: true) do %>
  ...
<% end %>

For the helper posts_path to be available, you must update your routes (ie config/routes.rb ) by adding resources :postsinstead of `post '/posts', to: 'posts#create'.

The above fixes the error, but your form will still not work since the content of the action createdoes not process the information you receive from the form (you are doing it in new, which is wrong). Following Rails (and REST ) standards, newit only responds with the verb GET, while it createresponds to POST. So adjust the controller PostsControllerto reflect the above; for instance:

class PostsController < ApplicationController
  # ...

  def create
    file = params[:exampleFormControlFile1]
    # código para guardar/procesar el archivo
  end
end

It's important to note that the code you currently have in createwon't work because it post_paramsdoesn't contain the form information, but it will work if you wrap that method (and assuming you've set up your model Postproperly); in fact that code is closer to what you should put in your application which, respecting the code of your form, could be something like this ¹ :

class PostsController < ApplicationController
  def new
    @post = Post.new
  end

  def create
    @post = Post.new(post_params)

    if @post.save
      redirect_to action: 'index'
    else
      render 'new'
    end
  end

  private
  def post_params
    params.permit(:exampleFormControlFile1)
  end
end

^{1 The form is assumed to be in the file new.html.erb , which would be correct by Rails standards.}