diff --git a/spring-web/src/main/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolver.java b/spring-web/src/main/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolver.java
index 59125b5ea0d..ac5569ab4bc 100644
--- a/spring-web/src/main/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolver.java
+++ b/spring-web/src/main/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolver.java
@@ -95,10 +95,12 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 	}
 
 	/**
-	 * Use this method for a reverse lookup from extension to MediaType.
+	 * Use this method for a reverse, case-insensitive MediaType lookup.
+	 * @param extension the extension to look up
 	 * @return a MediaType for the key or {@code null}
 	 */
 	protected MediaType lookupMediaType(String extension) {
+		extension = extension.toLowerCase(Locale.ENGLISH);
 		return this.mediaTypes.get(extension);
 	}
 
diff --git a/spring-web/src/test/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolverTests.java b/spring-web/src/test/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolverTests.java
index 4ea37728431..a999b9995b5 100644
--- a/spring-web/src/test/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolverTests.java
+++ b/spring-web/src/test/java/org/springframework/web/accept/MappingMediaTypeFileExtensionResolverTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -51,4 +51,14 @@ public class MappingMediaTypeFileExtensionResolverTests {
 		assertTrue(extensions.isEmpty());
 	}
 
+	// SPR-13747
+
+	@Test
+	public void lookupMediaTypeCaseInsensitive() {
+		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);
+		MappingMediaTypeFileExtensionResolver resolver = new MappingMediaTypeFileExtensionResolver(mapping);
+		MediaType mediaType = resolver.lookupMediaType("JSON");
+
+		assertEquals(mediaType, MediaType.APPLICATION_JSON);
+	}
 }