Fix default content-type for ResourceRegion HTTP responses

Prior to this commit, the `ResourceRegionHttpMessageConverter` would
rely on the default implementation of `getDefaultContentType` to guess
the default Content-Type of the resource region to be written to the
HTTP response. That implementation fetches the first media type
provided in the HTTP request "Accept" header.

This behavior is not correct when converting resources and this commits
aligns this converter with the `ResourceHttpMessageConverter` which uses
JAF to guess the correct Content-Type of the given resource, or just
returns "application/octet-stream" as a default value.

Issue: SPR-15041

spring-web/src/main/java/org/springframework/http/converter/ResourceRegionHttpMessageConverter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -29,7 +29,9 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpInputMessage;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
+import org.springframework.http.MediaTypeFactory;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
 import org.springframework.util.MimeTypeUtils;
 import org.springframework.util.StreamUtils;
 
@@ -42,6 +44,9 @@ import org.springframework.util.StreamUtils;
  */
 public class ResourceRegionHttpMessageConverter extends AbstractGenericHttpMessageConverter<Object> {
 
+	private static final boolean jafPresent = ClassUtils.isPresent(
+			"javax.activation.FileTypeMap", ResourceHttpMessageConverter.class.getClassLoader());
+
 	public ResourceRegionHttpMessageConverter() {
 		super(MediaType.ALL);
 	}
@@ -72,6 +77,23 @@ public class ResourceRegionHttpMessageConverter extends AbstractGenericHttpMessa
 		return null;
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	protected MediaType getDefaultContentType(Object object) {
+		if (jafPresent) {
+			if(object instanceof ResourceRegion) {
+				return MediaTypeFactory.getMediaType(((ResourceRegion) object).getResource());
+			}
+			else {
+				Collection<ResourceRegion> regions = (Collection<ResourceRegion>) object;
+				if(regions.size() > 0) {
+					return MediaTypeFactory.getMediaType(regions.iterator().next().getResource());
+				}
+			}
+		}
+		return MediaType.APPLICATION_OCTET_STREAM;
+	}
+
 	@Override
 	public boolean canWrite(Class<?> clazz, MediaType mediaType) {
 		return canWrite(clazz, null, mediaType);