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 3fee1520815..7acd788dd5f 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
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 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.
@@ -31,24 +31,28 @@ import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
 
 /**
- * An implementation of {@link MediaTypeFileExtensionResolver} that maintains a lookup
- * from extension to MediaType.
+ * An implementation of {@code MediaTypeFileExtensionResolver} that maintains
+ * lookups between file extensions and MediaTypes in both directions.
+ *
+ * <p>Initially created with a map of file extensions and media types.
+ * Subsequently sub-classes can use {@link #addMapping} to add more mappings.
  *
  * @author Rossen Stoyanchev
  * @since 3.2
  */
 public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExtensionResolver {
 
-	private final ConcurrentMap<String, MediaType> mediaTypes = new ConcurrentHashMap<String, MediaType>(64);
+	private final ConcurrentMap<String, MediaType> mediaTypes =
+			new ConcurrentHashMap<String, MediaType>(64);
 
-	private final MultiValueMap<MediaType, String> fileExtensions = new LinkedMultiValueMap<MediaType, String>();
+	private final MultiValueMap<MediaType, String> fileExtensions =
+			new LinkedMultiValueMap<MediaType, String>();
 
 	private final List<String> allFileExtensions = new LinkedList<String>();
 
 
 	/**
-	 * Create an instance with the given mappings between extensions and media types.
-	 * @throws IllegalArgumentException if a media type string cannot be parsed
+	 * Create an instance with the given map of file extensions and media types.
 	 */
 	public MappingMediaTypeFileExtensionResolver(Map<String, MediaType> mediaTypes) {
 		if (mediaTypes != null) {
@@ -61,10 +65,22 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 	}
 
 
+	protected List<MediaType> getAllMediaTypes() {
+		return new ArrayList<MediaType>(this.mediaTypes.values());
+	}
+
 	/**
-	 * Find the file extensions mapped to the given MediaType.
-	 * @return 0 or more extensions, never {@code null}
+	 * Map an extension to a MediaType. Ignore if extension already mapped.
 	 */
+	protected void addMapping(String extension, MediaType mediaType) {
+		MediaType previous = this.mediaTypes.putIfAbsent(extension, mediaType);
+		if (previous == null) {
+			this.fileExtensions.add(mediaType, extension);
+			this.allFileExtensions.add(extension);
+		}
+	}
+
+
 	@Override
 	public List<String> resolveFileExtensions(MediaType mediaType) {
 		List<String> fileExtensions = this.fileExtensions.get(mediaType);
@@ -76,27 +92,12 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 		return Collections.unmodifiableList(this.allFileExtensions);
 	}
 
-	protected List<MediaType> getAllMediaTypes() {
-		return new ArrayList<MediaType>(this.mediaTypes.values());
-	}
-
 	/**
-	 * Return the MediaType mapped to the given extension.
-	 * @return a MediaType for the key or {@code null}
+	 * Use this method for a reverse lookup from extension to MediaType.
+	 * @return a MediaType for the key, or {@code null} if none found
 	 */
 	protected MediaType lookupMediaType(String extension) {
-		return this.mediaTypes.get(extension);
-	}
-
-	/**
-	 * Map a MediaType to an extension or ignore if the extensions is already mapped.
-	 */
-	protected void addMapping(String extension, MediaType mediaType) {
-		MediaType previous = this.mediaTypes.putIfAbsent(extension, mediaType);
-		if (previous == null) {
-			this.fileExtensions.add(mediaType, extension);
-			this.allFileExtensions.add(extension);
-		}
+		return this.mediaTypes.get(extension.toLowerCase(Locale.ENGLISH));
 	}
 
 }
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..3bf9087e718 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.
@@ -29,6 +29,7 @@ import static org.junit.Assert.*;
  * Test fixture for {@link MappingMediaTypeFileExtensionResolver}.
  *
  * @author Rossen Stoyanchev
+ * @author Melissa Hartsock
  */
 public class MappingMediaTypeFileExtensionResolverTests {
 
@@ -51,4 +52,17 @@ public class MappingMediaTypeFileExtensionResolverTests {
 		assertTrue(extensions.isEmpty());
 	}
 
+	/**
+	 * Unit test for SPR-13747 - ensures that reverse lookup of media type from media
+	 * type key is case-insensitive.
+	 */
+	@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);
+	}
+
 }