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 72672f913eb..130f2bd5cf7 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
@@ -57,12 +57,7 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 	public MappingMediaTypeFileExtensionResolver(@Nullable Map<String, MediaType> mediaTypes) {
 		if (mediaTypes != null) {
 			Set<String> allFileExtensions = CollectionUtils.newHashSet(mediaTypes.size());
-			mediaTypes.forEach((extension, mediaType) -> {
-				String lowerCaseExtension = extension.toLowerCase(Locale.ROOT);
-				this.mediaTypes.put(lowerCaseExtension, mediaType);
-				addFileExtension(mediaType, lowerCaseExtension);
-				allFileExtensions.add(lowerCaseExtension);
-			});
+			mediaTypes.forEach((extension, mediaType) -> addMapping(extension.toLowerCase(Locale.ROOT), mediaType));
 			this.allFileExtensions.addAll(allFileExtensions);
 		}
 	}
@@ -82,16 +77,12 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 	protected void addMapping(String extension, MediaType mediaType) {
 		MediaType previous = this.mediaTypes.putIfAbsent(extension, mediaType);
 		if (previous == null) {
-			addFileExtension(mediaType, extension);
+			this.fileExtensions.computeIfAbsent(
+					mediaType, key -> new CopyOnWriteArrayList<>()).add(extension);
 			this.allFileExtensions.add(extension);
 		}
 	}
 
-	private void addFileExtension(MediaType mediaType, String extension) {
-		this.fileExtensions.computeIfAbsent(mediaType, key -> new CopyOnWriteArrayList<>())
-				.add(extension);
-	}
-
 
 	@Override
 	public List<String> resolveFileExtensions(MediaType mediaType) {
diff --git a/spring-web/src/test/java/org/springframework/web/accept/MappingContentNegotiationStrategyTests.java b/spring-web/src/test/java/org/springframework/web/accept/MappingContentNegotiationStrategyTests.java
index 70770d0ae00..14f9d5d40f3 100644
--- a/spring-web/src/test/java/org/springframework/web/accept/MappingContentNegotiationStrategyTests.java
+++ b/spring-web/src/test/java/org/springframework/web/accept/MappingContentNegotiationStrategyTests.java
@@ -23,9 +23,11 @@ import java.util.Map;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.MediaType;
+import org.springframework.web.HttpMediaTypeNotAcceptableException;
 import org.springframework.web.context.request.NativeWebRequest;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 /**
  * A test fixture with a test subclass of AbstractMappingContentNegotiationStrategy.
@@ -36,7 +38,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 class MappingContentNegotiationStrategyTests {
 
 	@Test
-	void resolveMediaTypes() throws Exception {
+	void resolveMediaTypesFromRegisteredExtensions() throws Exception {
 		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);
 		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("json", mapping);
 
@@ -47,27 +49,7 @@ class MappingContentNegotiationStrategyTests {
 	}
 
 	@Test
-	void resolveMediaTypesNoMatch() throws Exception {
-		Map<String, MediaType> mapping = null;
-		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("blah", mapping);
-
-		List<MediaType> mediaTypes = strategy.resolveMediaTypes(null);
-
-		assertThat(mediaTypes).isEqualTo(ContentNegotiationStrategy.MEDIA_TYPE_ALL_LIST);
-	}
-
-	@Test
-	void resolveMediaTypesNoKey() throws Exception {
-		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);
-		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy(null, mapping);
-
-		List<MediaType> mediaTypes = strategy.resolveMediaTypes(null);
-
-		assertThat(mediaTypes).isEqualTo(ContentNegotiationStrategy.MEDIA_TYPE_ALL_LIST);
-	}
-
-	@Test
-	void resolveMediaTypesHandleNoMatch() throws Exception {
+	void resolveMediaTypesViaMediaTypeFactory() throws Exception {
 		Map<String, MediaType> mapping = null;
 		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("xml", mapping);
 
@@ -77,6 +59,25 @@ class MappingContentNegotiationStrategyTests {
 		assertThat(mediaTypes.get(0).toString()).isEqualTo("application/xml");
 	}
 
+	@Test
+	void resolveMediaTypesUnknownKey() {
+		Map<String, MediaType> mapping = null;
+		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("blah", mapping);
+
+		assertThatThrownBy(() -> strategy.resolveMediaTypes(null))
+				.isInstanceOf(HttpMediaTypeNotAcceptableException.class);
+	}
+
+	@Test
+	void resolveMediaTypesNullKey() throws Exception {
+		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);
+		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy(null, mapping);
+
+		List<MediaType> mediaTypes = strategy.resolveMediaTypes(null);
+
+		assertThat(mediaTypes).isEqualTo(ContentNegotiationStrategy.MEDIA_TYPE_ALL_LIST);
+	}
+
 
 	private static class TestMappingContentNegotiationStrategy extends AbstractMappingContentNegotiationStrategy {
 
@@ -92,10 +93,6 @@ class MappingContentNegotiationStrategyTests {
 			return this.extension;
 		}
 
-		@Override
-		protected MediaType handleNoMatch(NativeWebRequest request, String mappingKey) {
-			return "xml".equals(mappingKey) ? MediaType.APPLICATION_XML : null;
-		}
 	}
 
 }
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 88c348d0973..b003a9825a7 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
@@ -68,13 +68,15 @@ class MappingMediaTypeFileExtensionResolverTests {
 	}
 
 	@Test
-	void allFileExtensions() {
-		Map<String, MediaType> mappings = new HashMap<>();
-		mappings.put("json", MediaType.APPLICATION_JSON);
-		mappings.put("JsOn", MediaType.APPLICATION_JSON);
-		mappings.put("jSoN", MediaType.APPLICATION_JSON);
+	void mappingsAreCaseInsensitive() {
+		Map<String, MediaType> map = new HashMap<>();
+		map.put("json", MediaType.APPLICATION_JSON);
+		map.put("JsOn", MediaType.APPLICATION_JSON);
+		map.put("jSoN", MediaType.APPLICATION_JSON);
 
-		MappingMediaTypeFileExtensionResolver resolver = new MappingMediaTypeFileExtensionResolver(mappings);
+		MappingMediaTypeFileExtensionResolver resolver = new MappingMediaTypeFileExtensionResolver(map);
 		assertThat(resolver.getAllFileExtensions()).containsExactly("json");
 	}
+
+
 }