app.rb

require 'rubygems'
require 'haml'
require 'sinatra'
require 'sinatra/flash'
require 'file_uploader'

enable :sessions

get '/' do
  haml :index
end

post '/upload' do
  FileUploader.create.save(params[:file]) do |path|
    flash[:upload] = "#{path} is saved."
  end

  redirect '/'
end

__END__

@@ layout
%html
  %head
    %title File Uploader
  %body
    = yield
    
@@ index
%h1 File Uploader

- if flash[:upload]
  %div{:style => 'color:green'}= flash[:upload]

%form{:action => '/upload', :method => 'post', :enctype => 'multipart/form-data'}
  %label{:for => 'file'} Select file:
  %input{:type => 'file', :id => 'file', :name => 'file'}
  %input{:type => 'submit', :value => 'Upload'}