Treat '+' as a safe MIME type character.

util/net/http_multipart_builder_test.cc

 // Copyright 2014 The Crashpad Authors. All rights reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
 //
 //     http://www.apache.org/licenses/LICENSE-2.0
 //
 // Unless required by applicable law or agreed to in writing, software
 // distributed under the License is distributed on an "AS IS" BASIS,
@@ skipping 239 matching lines @@
 
 TEST(HTTPMultipartBuilder, AssertUnsafeMIMEType) {
   HTTPMultipartBuilder builder;
   // Invalid and potentially dangerous:
   ASSERT_DEATH(builder.SetFileAttachment("", "", base::FilePath(), "\r\n"), "");
   ASSERT_DEATH(builder.SetFileAttachment("", "", base::FilePath(), "\""), "");
   ASSERT_DEATH(builder.SetFileAttachment("", "", base::FilePath(), "\x12"), "");
   ASSERT_DEATH(builder.SetFileAttachment("", "", base::FilePath(), "<>"), "");
   // Invalid but safe:
   builder.SetFileAttachment("", "", base::FilePath(), "0/totally/-invalid.pdf");
+  // Valid and safe:
+  builder.SetFileAttachment("", "", base::FilePath(), "application/xml+xhtml");
 }
 
 }  // namespace
 }  // namespace test
 }  // namespace crashpad