Fixes #25829 by changing the check away from instanceof. Now checking…

spring-web/src/main/java/org/springframework/web/multipart/support/RequestPartServletServerHttpRequest.java

@@ -20,8 +20,10 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.charset.Charset;
-
+import java.util.Collection;
+import javax.servlet.ServletException;
 import javax.servlet.http.HttpServletRequest;
+import javax.servlet.http.Part;
 
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
@@ -81,7 +83,13 @@ public HttpHeaders getHeaders() {
 
 	@Override
 	public InputStream getBody() throws IOException {
-		if (this.multipartRequest instanceof StandardMultipartHttpServletRequest) {
+		Collection<Part> parts = null;
+		try {
+			parts = this.multipartRequest.getParts();
+		} catch (ServletException e) {
+			//ignoring this as it might be a non multipart/form-data Content-Type
+		}
+		if (parts != null && !parts.isEmpty()) {
 			try {
 				return this.multipartRequest.getPart(this.partName).getInputStream();
 			}

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

@@ -22,6 +22,7 @@
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletRequestWrapper;
 
+import org.apache.catalina.Server;
 import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpHeaders;
@@ -32,6 +33,7 @@
 import org.springframework.web.multipart.MultipartFile;
 import org.springframework.web.testfixture.servlet.MockMultipartFile;
 import org.springframework.web.testfixture.servlet.MockMultipartHttpServletRequest;
+import org.springframework.web.testfixture.servlet.MockPart;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
@@ -137,4 +139,23 @@ public HttpHeaders getMultipartHeaders(String paramOrFileName) {
 		assertThat(result).isEqualTo(bytes);
 	}
 
+	@Test //#25829
+	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 = "Hello".getBytes();
+		MockPart mockPart = new MockPart("part", bytes);
+		mockPart.getHeaders().setContentType(MediaType.TEXT_PLAIN);
+		mockRequest.addPart(mockPart);
+		ServerHttpRequest request = new RequestPartServletServerHttpRequest(mockRequest, "part");
+		byte[] result = FileCopyUtils.copyToByteArray(request.getBody());
+		assertThat(result).isEqualTo(bytes);
+	}
+
 }