diff --git a/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeExtensionGuesser.php b/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeExtensionGuesser.php
index d5acb34094c1..c0f9140c800a 100644
--- a/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeExtensionGuesser.php
+++ b/src/Symfony/Component/HttpFoundation/File/MimeType/MimeTypeExtensionGuesser.php
@@ -808,6 +808,12 @@ class MimeTypeExtensionGuesser implements ExtensionGuesserInterface
      */
     public function guess($mimeType)
     {
-        return isset($this->defaultExtensions[$mimeType]) ? $this->defaultExtensions[$mimeType] : null;
+        if (isset($this->defaultExtensions[$mimeType])) {
+            return $this->defaultExtensions[$mimeType];
+        }
+
+        $lcMimeType = strtolower($mimeType);
+
+        return isset($this->defaultExtensions[$lcMimeType]) ? $this->defaultExtensions[$lcMimeType] : null;
     }
 }
diff --git a/src/Symfony/Component/HttpFoundation/Tests/File/Fixtures/case-sensitive-mime-type.xlsm b/src/Symfony/Component/HttpFoundation/Tests/File/Fixtures/case-sensitive-mime-type.xlsm
new file mode 100644
index 000000000000..94d85e6132c5
Binary files /dev/null and b/src/Symfony/Component/HttpFoundation/Tests/File/Fixtures/case-sensitive-mime-type.xlsm differ
diff --git a/src/Symfony/Component/HttpFoundation/Tests/File/UploadedFileTest.php b/src/Symfony/Component/HttpFoundation/Tests/File/UploadedFileTest.php
index 9c02b478edc0..5a37cda351f9 100644
--- a/src/Symfony/Component/HttpFoundation/Tests/File/UploadedFileTest.php
+++ b/src/Symfony/Component/HttpFoundation/Tests/File/UploadedFileTest.php
@@ -90,6 +90,19 @@ public function testGuessClientExtensionWithIncorrectMimeType()
         $this->assertEquals('jpeg', $file->guessClientExtension());
     }
 
+    public function testCaseSensitiveMimeType()
+    {
+        $file = new UploadedFile(
+            __DIR__.'/Fixtures/case-sensitive-mime-type.xlsm',
+            'test.xlsm',
+            'application/vnd.ms-excel.sheet.macroEnabled.12',
+            filesize(__DIR__.'/Fixtures/case-sensitive-mime-type.xlsm'),
+            null
+        );
+
+        $this->assertEquals('xlsm', $file->guessClientExtension());
+    }
+
     public function testErrorIsOkByDefault()
     {
         $file = new UploadedFile(