diff --git a/sharing/internal/base/BUILD b/sharing/internal/base/BUILD
index b21a9fce..c3187d9c 100644
--- a/sharing/internal/base/BUILD
+++ b/sharing/internal/base/BUILD
@@ -46,6 +46,7 @@ cc_test(
     timeout = "short",
     srcs = [
         "encode_test.cc",
+        "mime_test.cc",
     ],
     shard_count = 8,
     deps = [
diff --git a/sharing/internal/base/mime.cc b/sharing/internal/base/mime.cc
index ee76e5ff..d67f4ea0 100644
--- a/sharing/internal/base/mime.cc
+++ b/sharing/internal/base/mime.cc
@@ -17,6 +17,7 @@
 #include <string>
 
 #include "absl/container/flat_hash_map.h"
+#include "absl/strings/ascii.h"
 #include "absl/strings/string_view.h"
 
 namespace nearby {
@@ -25,6 +26,7 @@ namespace utils {
 // Refers to
 // http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types.
 std::string GetWellKnownMimeTypeFromExtension(absl::string_view extension) {
+  // The extension types in this map must be lower case.
   static absl::flat_hash_map<
       std::string,
       std::string>* mime_map = new absl::flat_hash_map<std::string,
@@ -1029,7 +1031,7 @@ std::string GetWellKnownMimeTypeFromExtension(absl::string_view extension) {
       {"zmm", "application/vnd.handheld-entertainment+xml"},
   });
 
-  return (*mime_map)[extension];
+  return (*mime_map)[absl::AsciiStrToLower(extension)];
 }  // NOLINT
 
 }  // namespace utils
diff --git a/sharing/internal/base/mime_test.cc b/sharing/internal/base/mime_test.cc
new file mode 100644
index 00000000..383b5df7
--- /dev/null
+++ b/sharing/internal/base/mime_test.cc
@@ -0,0 +1,47 @@
+// Copyright 2022 Google LLC
+//
+// 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
+//
+//     https://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,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "sharing/internal/base/mime.h"
+
+#include "gtest/gtest.h"
+
+namespace nearby::utils {
+namespace {
+
+TEST(MimeTest, GetWellKnownMimeTypeFromExtension) {
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("txt"), "text/plain");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("pdf"), "application/pdf");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("doc"),
+            "application/msword");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("docx"),
+            "application/vnd.openxmlformats-officedocument.wordprocessingml."
+            "document");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("xls"),
+            "application/vnd.ms-excel");
+}
+
+TEST(MimeTest, GetWellKnownMimeTypeFromExtensionCaseInsensitive) {
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("TXT"), "text/plain");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("PDF"), "application/pdf");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("DOC"), "application/msword");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("DOCX"),
+            "application/vnd.openxmlformats-officedocument.wordprocessingml."
+            "document");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("XLS"),
+            "application/vnd.ms-excel");
+  EXPECT_EQ(GetWellKnownMimeTypeFromExtension("PNG"), "image/png");
+}
+
+}  // namespace
+}  // namespace nearby::utils