Update from https://crrev.com/316786

net/base/mime_util.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef NET_BASE_MIME_UTIL_H__
 #define NET_BASE_MIME_UTIL_H__
 
+// This file defines MIME utility functions. All of them assume the MIME type
+// to be of the format specified by rfc2045. According to it, MIME types are
+// case strongly insensitive except parameter values, which may or may not be
+// case sensitive.
+//
+// These utilities perform a *case-sensitive* matching for  parameter values,
+// which may produce some false negatives. Except that, matching is
+// case-insensitive.
+//
+// All constants in mime_util.cc must be written in lower case, except parameter
+// values, which can be any case.
+
 #include <string>
 #include <vector>
 
 #include "base/files/file_path.h"
 #include "net/base/net_export.h"
 
 namespace net {
 
 // Get the mime type (if any) that is associated with the given file extension.
 // Returns true if a corresponding mime type exists.
@@ skipping 25 matching lines @@
 NET_EXPORT bool IsSupportedMediaMimeType(const std::string& mime_type);
 NET_EXPORT bool IsSupportedNonImageMimeType(const std::string& mime_type);
 NET_EXPORT bool IsUnsupportedTextMimeType(const std::string& mime_type);
 NET_EXPORT bool IsSupportedJavascriptMimeType(const std::string& mime_type);
 NET_EXPORT bool IsSupportedCertificateMimeType(const std::string& mime_type);
 
 // Convenience function.
 NET_EXPORT bool IsSupportedMimeType(const std::string& mime_type);
 
 // Returns true if this the mime_type_pattern matches a given mime-type.
-// Checks for absolute matching and wildcards.  mime-types should be in
-// lower case.
+// Checks for absolute matching and wildcards. MIME types are case insensitive.
 NET_EXPORT bool MatchesMimeType(const std::string& mime_type_pattern,
                                 const std::string& mime_type);
 
 // Returns true if the |type_string| is a correctly-formed mime type specifier
 // with no parameter, i.e. string that matches the following ABNF (see the
 // definition of content ABNF in RFC2045 and media-type ABNF httpbis p2
 // semantics).
 //
 //   token "/" token
 //
@@ skipping 53 matching lines @@
 //   support only a subset of codecs, but |codecs| was empty. Also returned if
 //   all the codecs in |codecs| are supported, but additional codec parameters
 //   were supplied (such as profile) for which the support cannot be decided.
 // * Returns IsNotSupported if either the |mime_type| is not supported or the
 //   |mime_type| is supported but at least one of the codecs within |codecs| is
 //   not supported for the |mime_type|.
 NET_EXPORT SupportsType IsSupportedStrictMediaMimeType(
     const std::string& mime_type,
     const std::vector<std::string>& codecs);
 
-// Get the extensions associated with the given mime type. This should be passed
-// in lower case. There could be multiple extensions for a given mime type, like
-// "html,htm" for "text/html", or "txt,text,html,..." for "text/*".
+// Get the extensions associated with the given mime type. There could be
+// multiple extensions for a given mime type, like "html,htm" for "text/html",
+// or "txt,text,html,..." for "text/*".
 // Note that we do not erase the existing elements in the the provided vector.
 // Instead, we append the result to it.
 NET_EXPORT void GetExtensionsForMimeType(
     const std::string& mime_type,
     std::vector<base::FilePath::StringType>* extensions);
 
 // Test only method that removes proprietary media types and codecs from the
 // list of supported MIME types and codecs. These types and codecs must be
 // removed to ensure consistent layout test results across all Chromium
 // variations.
@@ skipping 21 matching lines @@
                                            std::string* post_data);
 
 // Adds the final delimiter to a multi-part upload request.
 NET_EXPORT void AddMultipartFinalDelimiterForUpload(
     const std::string& mime_boundary,
     std::string* post_data);
 
 }  // namespace net
 
 #endif  // NET_BASE_MIME_UTIL_H__