Merge pull request #21723 from y-yagi/fix_deprecate_msg_in_mailers_controller

stop using deprecated method in mailers controller

railties/lib/rails/mailers_controller.rb

@@ -31,7 +31,7 @@ def preview
             raise AbstractController::ActionNotFound, "Email part '#{part_type}' not found in #{@preview.name}##{@email_action}"
           end
         else
-          @part = find_preferred_part(request.format, Mime::HTML, Mime::TEXT)
+          @part = find_preferred_part(request.format, Mime::Type[:HTML], Mime::Type[:TEXT])
           render action: 'email', layout: false, formats: %w[html]
         end
       else

railties/lib/rails/templates/rails/mailers/email.html.erb

@@ -95,8 +95,8 @@
     <% if @email.multipart? %>
       <dd>
         <select onchange="document.getElementsByName('messageBody')[0].src=this.options[this.selectedIndex].value;">
-          <option <%= request.format == Mime::HTML ? 'selected' : '' %> value="?part=text%2Fhtml">View as HTML email</option>
-          <option <%= request.format == Mime::TEXT ? 'selected' : '' %> value="?part=text%2Fplain">View as plain-text email</option>
+          <option <%= request.format == Mime::Type[:HTML] ? 'selected' : '' %> value="?part=text%2Fhtml">View as HTML email</option>
+          <option <%= request.format == Mime::Type[:TEXT] ? 'selected' : '' %> value="?part=text%2Fplain">View as plain-text email</option>
         </select>
       </dd>
     <% end %>