diff --git a/spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/FileExtensionContentTypeResolver.java b/spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/MappingContentTypeResolver.java
similarity index 69%
rename from spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/FileExtensionContentTypeResolver.java
rename to spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/MappingContentTypeResolver.java
index 8343c8da7cc..645beed0776 100644
--- a/spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/FileExtensionContentTypeResolver.java
+++ b/spring-web-reactive/src/main/java/org/springframework/web/reactive/accept/MappingContentTypeResolver.java
@@ -17,16 +17,17 @@
 package org.springframework.web.reactive.accept;
 
 import java.util.List;
+import java.util.Set;
 
 import org.springframework.http.MediaType;
 
 /**
- * An extension of {@link ContentTypeResolver} for a resolver that uses file
- * extensions and can expose file extension mappings.
+ * An extension of {@link ContentTypeResolver} that maintains a mapping between
+ * keys (e.g. file extension, query parameter) and media types.
  *
  * @author Rossen Stoyanchev
  */
-public interface FileExtensionContentTypeResolver extends ContentTypeResolver {
+public interface MappingContentTypeResolver extends ContentTypeResolver {
 
 	/**
 	 * Resolve the given media type to a list of path extensions.
@@ -34,12 +35,12 @@ public interface FileExtensionContentTypeResolver extends ContentTypeResolver {
 	 * @param mediaType the media type to resolve
 	 * @return a list of extensions or an empty list, never {@code null}
 	 */
-	List<String> getFileExtensions(MediaType mediaType);
+	Set<String> getKeysFor(MediaType mediaType);
 
 	/**
-	 * Return all registered file extensions.
-	 * @return a list of extensions or an empty list, never {@code null}
+	 * Return all registered keys (e.g. "json", "xml").
+	 * @return a list of keys or an empty list, never {@code null}
 	 */
-	List<String> getAllFileExtensions();
+	Set<String> getKeys();
 
 }
diff --git a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/condition/PatternsRequestCondition.java b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/condition/PatternsRequestCondition.java
index cba2fe75ef0..5c30eafe7c0 100644
--- a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/condition/PatternsRequestCondition.java
+++ b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/condition/PatternsRequestCondition.java
@@ -21,6 +21,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
 import java.util.Comparator;
+import java.util.HashSet;
 import java.util.Iterator;
 import java.util.LinkedHashSet;
 import java.util.List;
@@ -50,7 +51,7 @@ public final class PatternsRequestCondition extends AbstractRequestCondition<Pat
 
 	private final boolean useTrailingSlashMatch;
 
-	private final List<String> fileExtensions = new ArrayList<String>();
+	private final Set<String> fileExtensions = new HashSet<>();
 
 
 	/**
@@ -74,7 +75,7 @@ public final class PatternsRequestCondition extends AbstractRequestCondition<Pat
 	 */
 	public PatternsRequestCondition(String[] patterns, HttpRequestPathHelper pathHelper,
 			PathMatcher pathMatcher, boolean useSuffixPatternMatch, boolean useTrailingSlashMatch,
-			List<String> extensions) {
+			Set<String> extensions) {
 
 		this(asList(patterns), pathHelper, pathMatcher, useSuffixPatternMatch, useTrailingSlashMatch, extensions);
 	}
@@ -84,7 +85,7 @@ public final class PatternsRequestCondition extends AbstractRequestCondition<Pat
 	 */
 	private PatternsRequestCondition(Collection<String> patterns, HttpRequestPathHelper pathHelper,
 			PathMatcher pathMatcher, boolean useSuffixPatternMatch, boolean useTrailingSlashMatch,
-			List<String> fileExtensions) {
+			Set<String> fileExtensions) {
 
 		this.patterns = Collections.unmodifiableSet(prependLeadingSlash(patterns));
 		this.pathHelper = (pathHelper != null ? pathHelper : new HttpRequestPathHelper());
diff --git a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/RequestMappingInfo.java b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/RequestMappingInfo.java
index 15f91ea04d6..bebceeb3e2e 100644
--- a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/RequestMappingInfo.java
+++ b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/RequestMappingInfo.java
@@ -16,13 +16,13 @@
 
 package org.springframework.web.reactive.result.method;
 
-import java.util.List;
+import java.util.Set;
 
 import org.springframework.util.PathMatcher;
 import org.springframework.util.StringUtils;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.reactive.accept.ContentTypeResolver;
-import org.springframework.web.reactive.accept.FileExtensionContentTypeResolver;
+import org.springframework.web.reactive.accept.MappingContentTypeResolver;
 import org.springframework.web.reactive.result.condition.ConsumesRequestCondition;
 import org.springframework.web.reactive.result.condition.HeadersRequestCondition;
 import org.springframework.web.reactive.result.condition.ParamsRequestCondition;
@@ -576,11 +576,11 @@ public final class RequestMappingInfo implements RequestCondition<RequestMapping
 		 * {@code registeredSuffixPatternMatch=true}, the extensions are obtained
 		 * from the configured {@code contentTypeResolver}.
 		 */
-		public List<String> getFileExtensions() {
+		public Set<String> getFileExtensions() {
 			ContentTypeResolver resolver = getContentTypeResolver();
 			if (useRegisteredSuffixPatternMatch() && resolver != null) {
-				if (resolver instanceof FileExtensionContentTypeResolver) {
-					return ((FileExtensionContentTypeResolver) resolver).getAllFileExtensions();
+				if (resolver instanceof MappingContentTypeResolver) {
+					return ((MappingContentTypeResolver) resolver).getKeys();
 				}
 			}
 			return null;
diff --git a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMapping.java b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMapping.java
index 96ec3b6c6e5..26ae11b43aa 100644
--- a/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMapping.java
+++ b/spring-web-reactive/src/main/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMapping.java
@@ -18,7 +18,7 @@ package org.springframework.web.reactive.result.method.annotation;
 
 import java.lang.reflect.AnnotatedElement;
 import java.lang.reflect.Method;
-import java.util.List;
+import java.util.Set;
 
 import org.springframework.context.EmbeddedValueResolverAware;
 import org.springframework.core.annotation.AnnotatedElementUtils;
@@ -149,7 +149,7 @@ public class RequestMappingHandlerMapping extends RequestMappingInfoHandlerMappi
 	/**
 	 * Return the file extensions to use for suffix pattern matching.
 	 */
-	public List<String> getFileExtensions() {
+	public Set<String> getFileExtensions() {
 		return this.config.getFileExtensions();
 	}
 
diff --git a/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/condition/PatternsRequestConditionTests.java b/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/condition/PatternsRequestConditionTests.java
index 715b247425c..4720f94b6ad 100644
--- a/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/condition/PatternsRequestConditionTests.java
+++ b/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/condition/PatternsRequestConditionTests.java
@@ -19,7 +19,7 @@ package org.springframework.web.reactive.result.condition;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.Collections;
-import java.util.List;
+import java.util.Set;
 
 import org.junit.Test;
 
@@ -131,7 +131,7 @@ public class PatternsRequestConditionTests {
 	@Test
 	public void matchSuffixPatternUsingFileExtensions() throws Exception {
 		String[] patterns = new String[] {"/jobs/{jobName}"};
-		List<String> extensions = Collections.singletonList("json");
+		Set<String> extensions = Collections.singleton("json");
 		PatternsRequestCondition condition = new PatternsRequestCondition(patterns, null, null, true, false, extensions);
 
 		ServerWebExchange exchange = createExchange("/jobs/my.job");
@@ -150,7 +150,7 @@ public class PatternsRequestConditionTests {
 	@Test
 	public void matchSuffixPatternUsingFileExtensions2() throws Exception {
 		PatternsRequestCondition condition1 = new PatternsRequestCondition(
-				new String[] {"/prefix"}, null, null, true, false, Collections.singletonList("json"));
+				new String[] {"/prefix"}, null, null, true, false, Collections.singleton("json"));
 
 		PatternsRequestCondition condition2 = new PatternsRequestCondition(
 				new String[] {"/suffix"}, null, null, true, false, null);
diff --git a/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMappingTests.java b/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMappingTests.java
index f8feef526d1..3afe014a614 100644
--- a/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMappingTests.java
+++ b/spring-web-reactive/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingHandlerMappingTests.java
@@ -39,7 +39,7 @@ import org.springframework.web.bind.annotation.PutMapping;
 import org.springframework.web.bind.annotation.RequestMapping;
 import org.springframework.web.bind.annotation.RequestMethod;
 import org.springframework.web.context.support.StaticWebApplicationContext;
-import org.springframework.web.reactive.accept.FileExtensionContentTypeResolver;
+import org.springframework.web.reactive.accept.MappingContentTypeResolver;
 import org.springframework.web.reactive.result.method.RequestMappingInfo;
 
 import static org.junit.Assert.assertArrayEquals;
@@ -73,8 +73,8 @@ public class RequestMappingHandlerMappingTests {
 		assertTrue(this.handlerMapping.useSuffixPatternMatch());
 		assertTrue(this.handlerMapping.useRegisteredSuffixPatternMatch());
 
-		FileExtensionContentTypeResolver contentTypeResolver = mock(FileExtensionContentTypeResolver.class);
-		when(contentTypeResolver.getAllFileExtensions()).thenReturn(Collections.singletonList("json"));
+		MappingContentTypeResolver contentTypeResolver = mock(MappingContentTypeResolver.class);
+		when(contentTypeResolver.getKeys()).thenReturn(Collections.singleton("json"));
 
 		this.handlerMapping.setContentTypeResolver(contentTypeResolver);
 		this.handlerMapping.afterPropertiesSet();
@@ -86,8 +86,8 @@ public class RequestMappingHandlerMappingTests {
 
 	@Test
 	public void useRegisteredSuffixPatternMatchInitialization() {
-		FileExtensionContentTypeResolver contentTypeResolver = mock(FileExtensionContentTypeResolver.class);
-		when(contentTypeResolver.getAllFileExtensions()).thenReturn(Collections.singletonList("json"));
+		MappingContentTypeResolver contentTypeResolver = mock(MappingContentTypeResolver.class);
+		when(contentTypeResolver.getKeys()).thenReturn(Collections.singleton("json"));
 
 		final Set<String> actualExtensions = new HashSet<>();
 		RequestMappingHandlerMapping localHandlerMapping = new RequestMappingHandlerMapping() {