diff --git a/actionmailer/lib/action_mailer/base.rb b/actionmailer/lib/action_mailer/base.rb
index db4589ee8f4a322ccf8ac595ab669ecb0b6ccedc..b77409b64bd424ea140e7cdca03c92e76870a7a9 100644
--- a/actionmailer/lib/action_mailer/base.rb
+++ b/actionmailer/lib/action_mailer/base.rb
@@ -479,7 +479,7 @@ def create!(method_name, *parameters) #:nodoc:
             )
           end
           unless @parts.empty?
-            @content_type = "multipart/alternative"
+            @content_type = "multipart/alternative" if @content_type !~ /^multipart/
             @parts = sort_parts(@parts, @implicit_parts_order)
           end
         end
diff --git a/actionmailer/test/mail_layout_test.rb b/actionmailer/test/mail_layout_test.rb
index c185bd5acdb3cdc6db49844e88e47d09ece97f96..50901f52ece63c177b6b4035fd111cc883751f62 100644
--- a/actionmailer/test/mail_layout_test.rb
+++ b/actionmailer/test/mail_layout_test.rb
@@ -21,10 +21,12 @@ def nolayout(recipient)
     body       render(:inline => "Hello, <%= @world %>", :layout => false, :body => { :world => "Earth" })
   end
 
-  def multipart(recipient)
+  def multipart(recipient, type = nil)
     recipients recipient
     subject    "You have a mail"
     from       "tester@example.com"
+
+    content_type(type) if type
   end
 end
 
@@ -64,6 +66,19 @@ def test_should_pickup_default_layout
 
   def test_should_pickup_multipart_layout
     mail = AutoLayoutMailer.create_multipart(@recipient)
+    assert_equal "multipart/alternative", mail.content_type
+    assert_equal 2, mail.parts.size
+
+    assert_equal 'text/plain', mail.parts.first.content_type
+    assert_equal "text/plain layout - text/plain multipart", mail.parts.first.body
+
+    assert_equal 'text/html', mail.parts.last.content_type
+    assert_equal "Hello from layout text/html multipart", mail.parts.last.body
+  end
+
+  def test_should_pickup_multipartmixed_layout
+    mail = AutoLayoutMailer.create_multipart(@recipient, "multipart/mixed")
+    assert_equal "multipart/mixed", mail.content_type
     assert_equal 2, mail.parts.size
 
     assert_equal 'text/plain', mail.parts.first.content_type
@@ -73,6 +88,19 @@ def test_should_pickup_multipart_layout
     assert_equal "Hello from layout text/html multipart", mail.parts.last.body
   end
 
+  def test_should_fix_multipart_layout
+    mail = AutoLayoutMailer.create_multipart(@recipient, "text/plain")
+    assert_equal "multipart/alternative", mail.content_type
+    assert_equal 2, mail.parts.size
+
+    assert_equal 'text/plain', mail.parts.first.content_type
+    assert_equal "text/plain layout - text/plain multipart", mail.parts.first.body
+
+    assert_equal 'text/html', mail.parts.last.content_type
+    assert_equal "Hello from layout text/html multipart", mail.parts.last.body
+  end
+
+
   def test_should_pickup_layout_given_to_render
     mail = AutoLayoutMailer.create_spam(@recipient)
     assert_equal "Spammer layout Hello, Earth", mail.body.strip