escape filename in the Content-Disposition header

According the multipart form data spec in WHATWG living standard.
Ref: https://html.spec.whatwg.org/#multipart-form-data

lib/sinatra/base.rb

@@ -381,16 +381,23 @@ module Sinatra
       response['Content-Type'] = mime_type
     end
 
+    # https://html.spec.whatwg.org/#multipart-form-data
+    MULTIPART_FORM_DATA_REPLACEMENT_TABLE = {
+      '"'  => '%22',
+      "\r" => '%0D',
+      "\n" => '%0A'
+    }.freeze
+
     # Set the Content-Disposition to "attachment" with the specified filename,
     # instructing the user agents to prompt to save.
     def attachment(filename = nil, disposition = :attachment)
       response['Content-Disposition'] = disposition.to_s.dup
-      if filename
-        params = '; filename="%s"' % File.basename(filename)
-        response['Content-Disposition'] << params
-        ext = File.extname(filename)
-        content_type(ext) unless response['Content-Type'] or ext.empty?
-      end
+      return unless filename
+
+      params = format('; filename="%s"', File.basename(filename).gsub(/["\r\n]/, MULTIPART_FORM_DATA_REPLACEMENT_TABLE))
+      response['Content-Disposition'] << params
+      ext = File.extname(filename)
+      content_type(ext) unless response['Content-Type'] || ext.empty?
     end
 
     # Use the contents of the file at +path+ as the response body.

test/helpers_test.rb

@@ -781,6 +781,18 @@ class HelpersTest < Minitest::Test
       assert_equal '<sinatra></sinatra>', body
     end
 
+    it 'escapes filename in the Content-Disposition header according to the multipart form data spec in WHATWG living standard' do
+      mock_app do
+        get('/attachment') do
+          attachment "test.xml\";\r\next=.txt"
+          response.write("<sinatra></sinatra>")
+        end
+      end
+
+      get '/attachment'
+      assert_equal 'attachment; filename="test.xml%22;%0D%0Aext=.txt"', response['Content-Disposition']
+      assert_equal '<sinatra></sinatra>', body
+    end
   end
 
   describe 'send_file' do