diff --git a/Gemfile.lock b/Gemfile.lock
index c7caa09077a17..5bbce932bc477 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -174,14 +174,12 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
     loofah (2.0.3)
       nokogiri (>= 1.5.9)
-    mail (2.6.6)
-      mime-types (>= 1.16, < 4)
+    mail (2.7.0)
+      mini_mime (>= 0.1.1)
     metaclass (0.0.4)
     method_source (0.8.2)
-    mime-types (3.1)
-      mime-types-data (~> 3.2015)
-    mime-types-data (3.2016.0521)
-    mini_portile2 (2.1.0)
+    mini_mime (0.1.3)
+    mini_portile2 (2.2.0)
     minitest (5.3.3)
     mocha (0.14.0)
       metaclass (~> 0.0.1)
diff --git a/actionmailer/test/base_test.rb b/actionmailer/test/base_test.rb
index 50f2c71737939..d370c88a00cb0 100644
--- a/actionmailer/test/base_test.rb
+++ b/actionmailer/test/base_test.rb
@@ -315,22 +315,21 @@ def welcome
 
   test "implicit multipart with attachments creates nested parts" do
     email = BaseMailer.implicit_multipart(attachments: true)
-    assert_equal("application/pdf", email.parts[0].mime_type)
-    assert_equal("multipart/alternative", email.parts[1].mime_type)
-    assert_equal("text/plain", email.parts[1].parts[0].mime_type)
-    assert_equal("TEXT Implicit Multipart", email.parts[1].parts[0].body.encoded)
-    assert_equal("text/html", email.parts[1].parts[1].mime_type)
-    assert_equal("HTML Implicit Multipart", email.parts[1].parts[1].body.encoded)
+    assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+    multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+    assert_equal("text/plain", multipart.parts[0].mime_type)
+    assert_equal("TEXT Implicit Multipart", multipart.parts[0].body.encoded)
+    assert_equal("text/html", multipart.parts[1].mime_type)
+    assert_equal("HTML Implicit Multipart", multipart.parts[1].body.encoded)
   end
 
   test "implicit multipart with attachments and sort order" do
     order = ["text/html", "text/plain"]
     with_default BaseMailer, parts_order: order do
       email = BaseMailer.implicit_multipart(attachments: true)
-      assert_equal("application/pdf", email.parts[0].mime_type)
-      assert_equal("multipart/alternative", email.parts[1].mime_type)
-      assert_equal("text/plain", email.parts[1].parts[1].mime_type)
-      assert_equal("text/html", email.parts[1].parts[0].mime_type)
+      assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+      multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+      assert_equal(%w[ text/html text/plain ], multipart.parts.map(&:mime_type).sort)
     end
   end
 
@@ -421,12 +420,12 @@ def welcome
 
   test "explicit multipart with attachments creates nested parts" do
     email = BaseMailer.explicit_multipart(attachments: true)
-    assert_equal("application/pdf", email.parts[0].mime_type)
-    assert_equal("multipart/alternative", email.parts[1].mime_type)
-    assert_equal("text/plain", email.parts[1].parts[0].mime_type)
-    assert_equal("TEXT Explicit Multipart", email.parts[1].parts[0].body.encoded)
-    assert_equal("text/html", email.parts[1].parts[1].mime_type)
-    assert_equal("HTML Explicit Multipart", email.parts[1].parts[1].body.encoded)
+    assert_equal(%w[ application/pdf multipart/alternative ], email.parts.map(&:mime_type).sort)
+    multipart = email.parts.detect { |p| p.mime_type == "multipart/alternative" }
+    assert_equal("text/plain", multipart.parts[0].mime_type)
+    assert_equal("TEXT Explicit Multipart", multipart.parts[0].body.encoded)
+    assert_equal("text/html", multipart.parts[1].mime_type)
+    assert_equal("HTML Explicit Multipart", multipart.parts[1].body.encoded)
   end
 
   test "explicit multipart with templates" do
diff --git a/actionmailer/test/test_helper_test.rb b/actionmailer/test/test_helper_test.rb
index 0a4bc75d3e381..06399b05114a9 100644
--- a/actionmailer/test/test_helper_test.rb
+++ b/actionmailer/test/test_helper_test.rb
@@ -40,7 +40,7 @@ def test_charset_is_utf_8
   end
 
   def test_encode
-    assert_equal '=?UTF-8?Q?This_is_=E3=81=82_string?=', encode('This is あ string')
+    assert_equal "This is あ string", Mail::Encodings.q_value_decode(encode("This is あ string"))
   end
 
   def test_read_fixture
diff --git a/guides/bug_report_templates/action_controller_master.rb b/guides/bug_report_templates/action_controller_master.rb
index 8522694b06be4..a3128d92a5add 100644
--- a/guides/bug_report_templates/action_controller_master.rb
+++ b/guides/bug_report_templates/action_controller_master.rb
@@ -1,3 +1,5 @@
+gem "bundler", "< 1.16"
+
 begin
   require 'bundler/inline'
 rescue LoadError => e
diff --git a/guides/bug_report_templates/active_record_master.rb b/guides/bug_report_templates/active_record_master.rb
index 00653471313f0..d38e9f509b5bb 100644
--- a/guides/bug_report_templates/active_record_master.rb
+++ b/guides/bug_report_templates/active_record_master.rb
@@ -1,3 +1,5 @@
+gem "bundler", "< 1.16"
+
 begin
   require 'bundler/inline'
 rescue LoadError => e
diff --git a/guides/bug_report_templates/generic_master.rb b/guides/bug_report_templates/generic_master.rb
index c8f1bc1c1ab83..eeb27a1828dd4 100644
--- a/guides/bug_report_templates/generic_master.rb
+++ b/guides/bug_report_templates/generic_master.rb
@@ -1,3 +1,5 @@
+gem "bundler", "< 1.16"
+
 begin
   require 'bundler/inline'
 rescue LoadError => e