Attempt fallback Part resolution even without StandardMultipartHttpServletRequest

Closes gh-25829

spring-web/src/test/java/org/springframework/web/multipart/support/RequestPartServletServerHttpRequestTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -32,11 +32,13 @@ import org.springframework.mock.web.test.MockMultipartFile;
 import org.springframework.mock.web.test.MockMultipartHttpServletRequest;
 import org.springframework.util.FileCopyUtils;
 import org.springframework.web.multipart.MultipartFile;
+import org.springframework.mock.web.test.MockPart;
 
 import static org.junit.Assert.*;
 
 /**
  * @author Rossen Stoyanchev
+ * @author Juergen Hoeller
  */
 public class RequestPartServletServerHttpRequestTests {
 
@@ -137,4 +139,26 @@ public class RequestPartServletServerHttpRequestTests {
 		assertArrayEquals(bytes, result);
 	}
 
+	@Test
+	public void getBodyViaRequestPart() throws Exception {
+		MockMultipartHttpServletRequest mockRequest = new MockMultipartHttpServletRequest() {
+			@Override
+			public HttpHeaders getMultipartHeaders(String paramOrFileName) {
+				HttpHeaders headers = new HttpHeaders();
+				headers.setContentType(MediaType.APPLICATION_OCTET_STREAM);
+				return headers;
+			}
+		};
+
+		byte[] bytes = "content".getBytes("UTF-8");
+		MockPart mockPart = new MockPart("part", bytes);
+		mockPart.getHeaders().setContentType(MediaType.APPLICATION_JSON);
+		mockRequest.addPart(mockPart);
+		mockRequest.addPart(mockPart);
+		ServerHttpRequest request = new RequestPartServletServerHttpRequest(mockRequest, "part");
+
+		byte[] result = FileCopyUtils.copyToByteArray(request.getBody());
+		assertArrayEquals(bytes, result);
+	}
+
 }