diff --git a/spring-web/src/main/java/org/springframework/web/accept/AbstractMappingContentNegotiationStrategy.java b/spring-web/src/main/java/org/springframework/web/accept/AbstractMappingContentNegotiationStrategy.java
index 3b48770ce90..6793856f7c2 100644
--- a/spring-web/src/main/java/org/springframework/web/accept/AbstractMappingContentNegotiationStrategy.java
+++ b/spring-web/src/main/java/org/springframework/web/accept/AbstractMappingContentNegotiationStrategy.java
@@ -148,9 +148,11 @@ public abstract class AbstractMappingContentNegotiationStrategy extends MappingM
 			throws HttpMediaTypeNotAcceptableException {
 
 		if (!isUseRegisteredExtensionsOnly()) {
-			Optional<MediaType> mediaType = MediaTypeFactory.getMediaType("file." + key);
-			if (mediaType.isPresent()) {
-				return mediaType.get();
+			if (key.indexOf('.') == -1) {
+				Optional<MediaType> mediaType = MediaTypeFactory.getMediaType("file." + key);
+				if (mediaType.isPresent()) {
+					return mediaType.get();
+				}
 			}
 		}
 		if (isIgnoreUnknownExtensions()) {
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 130f2bd5cf7..28680bdebf9 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
@@ -57,7 +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) -> addMapping(extension.toLowerCase(Locale.ROOT), mediaType));
+			mediaTypes.forEach(this::addMapping);
 			this.allFileExtensions.addAll(allFileExtensions);
 		}
 	}
@@ -74,12 +74,12 @@ public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExten
 	/**
 	 * 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);
+	protected void addMapping(String key, MediaType mediaType) {
+		key = key.toLowerCase(Locale.ROOT);
+		MediaType previous = this.mediaTypes.putIfAbsent(key, mediaType);
 		if (previous == null) {
-			this.fileExtensions.computeIfAbsent(
-					mediaType, key -> new CopyOnWriteArrayList<>()).add(extension);
-			this.allFileExtensions.add(extension);
+			this.fileExtensions.computeIfAbsent(mediaType, k -> new CopyOnWriteArrayList<>()).add(key);
+			this.allFileExtensions.add(key);
 		}
 	}
 
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 14f9d5d40f3..8bea3227389 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
@@ -49,7 +49,7 @@ class MappingContentNegotiationStrategyTests {
 	}
 
 	@Test
-	void resolveMediaTypesViaMediaTypeFactory() throws Exception {
+	void resolveMediaTypesViaFactory() throws Exception {
 		Map<String, MediaType> mapping = null;
 		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("xml", mapping);
 
@@ -68,6 +68,15 @@ class MappingContentNegotiationStrategyTests {
 				.isInstanceOf(HttpMediaTypeNotAcceptableException.class);
 	}
 
+	@Test
+	void resolveMediaTypesInvalidKey() {
+		Map<String, MediaType> mapping = null;
+		TestMappingContentNegotiationStrategy strategy = new TestMappingContentNegotiationStrategy("not.json", mapping);
+
+		assertThatThrownBy(() -> strategy.resolveMediaTypes(null))
+				.isInstanceOf(HttpMediaTypeNotAcceptableException.class);
+	}
+
 	@Test
 	void resolveMediaTypesNullKey() throws Exception {
 		Map<String, MediaType> mapping = Collections.singletonMap("json", MediaType.APPLICATION_JSON);
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 b003a9825a7..652b87b6a78 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
@@ -75,6 +75,8 @@ class MappingMediaTypeFileExtensionResolverTests {
 		map.put("jSoN", MediaType.APPLICATION_JSON);
 
 		MappingMediaTypeFileExtensionResolver resolver = new MappingMediaTypeFileExtensionResolver(map);
+		map.forEach(resolver::addMapping);
+
 		assertThat(resolver.getAllFileExtensions()).containsExactly("json");
 	}