Ruby on Rails: read user submited .txt file into variable

I'm building a ruby on rails app that lets the user pick a file from their hard drive, and then writes specific lines from the text file into a database. I've built the form, but now I don't know how to get the content of the file into a variable, format it and then write it into the database.

I have a database table named drafts with colums set_1, set_2, set_3. I want to write line 13(from the text file) into set_1, line 165 into set_2 and line 317 into set_3. I also want to format the lines before writing them. In the file they look like this ------ FRF ------ and I only want FRF.

I've spent a lot of time searching stackoverflow and rubyonrails guides but have a hard time figuring this out. I'm very new to ruby and rails in general, so any help is appreciated.

Here's my controller:

class DraftsController < ApplicationController
    def new
        @page_title = "Upload MTGO Draft"
        @draft = Draft.new  
    end

    def create
        @draft = Draft.new(draft_params)
        if @draft.save
            flash[:notice] = "Draft Saved!"
            redirect_to drafts_show_path
        else
            render "new"
        end
    end

    def index
        @page_title = "MTGO Draft"
    end

    def show
        @page_title = "MTGO Draft Replayer"
    end 

    def search
        @page_title = "Search MTGO Draft"
    end

    def destroy

    end

    private
        def draft_params
            params.require(:draft).permit(:name)
        end
end

and my new.html.erb:

<%= form_for @draft do |f| %>
    <div class="form-group">
        Draft Name:<%= f.text_field :name, class: 'form-control' %><br />
        <%= f.submit "Create" %>
    </div>
<% end %>

<%= form_for @draft do |f| %>
    <div class="form-group">
        Draft Log: <%= f.file_field :file %>
    </div>
<% end %>

If you need more info or want to get into the whole app more he's a gitHub repository.

Thanks for the help.