diff --git lib/sinatra/base.rb lib/sinatra/base.rb
index 819dd37..d3ff4e5 100644
--- lib/sinatra/base.rb
+++ lib/sinatra/base.rb
@@ -151,7 +151,7 @@ module Sinatra
       if calculate_content_length?
         # if some other code has already set Content-Length, don't muck with it
         # currently, this would be the static file-handler
-        headers["Content-Length"] = body.inject(0) { |l, p| l + Rack::Utils.bytesize(p) }.to_s
+        headers["Content-Length"] = body.inject(0) { |l, p| l + p.bytesize }.to_s
       end
 
       [status.to_i, headers, result]
@@ -240,7 +240,11 @@ module Sinatra
         def block.each; yield(call) end
         response.body = block
       elsif value
-        headers.delete 'Content-Length' unless request.head? || value.is_a?(Rack::File) || value.is_a?(Stream)
+        # Rack 2.0 returns a Rack::File::Iterator here instead of
+        # Rack::File as it was in the previous API.
+        unless request.head? || value.is_a?(Rack::File::Iterator) || value.is_a?(Stream)
+          headers.delete 'Content-Length'
+        end
         response.body = value
       else
         response.body
@@ -363,13 +367,13 @@ module Sinatra
 
       last_modified opts[:last_modified] if opts[:last_modified]
 
-      file      = Rack::File.new nil
-      file.path = path
-      result    = file.serving env
+      file   = Rack::File.new(File.dirname(settings.app_file))
+      result = file.serving(request, path)
+
       result[1].each { |k,v| headers[k] ||= v }
       headers['Content-Length'] = result[1]['Content-Length']
       opts[:status] &&= Integer(opts[:status])
-      halt opts[:status] || result[0], result[2]
+      halt (opts[:status] || result[0]), result[2]
     rescue Errno::ENOENT
       not_found
     end
@@ -1065,6 +1069,7 @@ module Sinatra
     # Run the block with 'throw :halt' support and apply result to the response.
     def invoke
       res = catch(:halt) { yield }
+
       res = [res] if Fixnum === res or String === res
       if Array === res and Fixnum === res.first
         res = res.dup
diff --git lib/sinatra/show_exceptions.rb lib/sinatra/show_exceptions.rb
index 2e3069f..2988cc4 100644
--- lib/sinatra/show_exceptions.rb
+++ lib/sinatra/show_exceptions.rb
@@ -44,7 +44,7 @@ module Sinatra
         500,
         {
           "Content-Type" => content_type,
-          "Content-Length" => Rack::Utils.bytesize(body.join).to_s
+          "Content-Length" => body.join.bytesize.to_s
         },
         body
       ]
diff --git test/settings_test.rb test/settings_test.rb
index 7ec03c6..1b135f5 100644
--- test/settings_test.rb
+++ test/settings_test.rb
@@ -244,7 +244,7 @@ class SettingsTest < Minitest::Test
       get '/'
       assert_equal 500, status
       assert body.include?("StandardError")
-      assert body.include?("<code>show_exceptions</code> setting")
+      assert body.include?("<code>Rack::ShowExceptions</code>")
     end
 
     it 'does not override app-specified error handling when set to :after_handler' do