Return null from MockServletContext.getMimeType for unknown type

ServletContext.getMimeType() returns `null` for unknown mime types; not
`application/octet-stream`.

Issue: SPR-14908

Author: poutsma

spring-test/src/main/java/org/springframework/mock/web/MockServletContext.java

@@ -50,6 +50,7 @@
 import org.springframework.http.MediaTypeFactory;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.MimeType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.web.util.WebUtils;
@@ -267,7 +268,9 @@ public String getMimeType(String filePath) {
 			return this.mimeTypes.get(extension).toString();
 		}
 		else {
-			return MediaTypeFactory.getMediaType(filePath).orElse(MediaType.APPLICATION_OCTET_STREAM).toString();
+			return MediaTypeFactory.getMediaType(filePath).
+					map(MimeType::toString)
+					.orElse(null);
 		}
 	}
 

spring-test/src/test/java/org/springframework/mock/web/MockServletContextTests.java

@@ -82,6 +82,7 @@ public void registerContextAndGetContext() {
 	public void getMimeType() {
 		assertEquals("text/html", sc.getMimeType("test.html"));
 		assertEquals("image/gif", sc.getMimeType("test.gif"));
+		assertNull(sc.getMimeType("test.foobar"));
 	}
 
 	/**

spring-web/src/test/java/org/springframework/mock/web/test/MockServletContext.java

@@ -50,6 +50,7 @@
 import org.springframework.http.MediaTypeFactory;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.MimeType;
 import org.springframework.util.ObjectUtils;
 import org.springframework.util.StringUtils;
 import org.springframework.web.util.WebUtils;
@@ -266,7 +267,9 @@ public String getMimeType(String filePath) {
 			return this.mimeTypes.get(extension).toString();
 		}
 		else {
-			return MediaTypeFactory.getMediaType(filePath).orElse(MediaType.APPLICATION_OCTET_STREAM).toString();
+			return MediaTypeFactory.getMediaType(filePath).
+					map(MimeType::toString)
+					.orElse(null);
 		}
 	}