Consistently set content type of the request with multipart

Closes gh-33232

spring-test/src/main/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilder.java

@@ -58,6 +58,7 @@ public abstract class AbstractMockMultipartHttpServletRequestBuilder<B extends A
 
 	protected AbstractMockMultipartHttpServletRequestBuilder(HttpMethod httpMethod) {
 		super(httpMethod);
+		super.contentType(MediaType.MULTIPART_FORM_DATA);
 	}
 
 	/**

spring-test/src/test/java/org/springframework/test/web/servlet/assertj/MockMvcTesterIntegrationTests.java

@@ -232,6 +232,13 @@ public class MockMvcTesterIntegrationTests {
 					"name": "test"
 				}""".getBytes(StandardCharsets.UTF_8));
 
+		@Test
+		void multipartSetsContentType() {
+			assertThat(mvc.put().uri("/multipart-put").multipart().file(file).file(JSON_PART_FILE))
+					.request().satisfies(request -> assertThat(request.getContentType())
+							.isEqualTo(MediaType.MULTIPART_FORM_DATA_VALUE));
+		}
+
 		@Test
 		void multipartWithPut() {
 			assertThat(mvc.put().uri("/multipart-put").multipart().file(file).file(JSON_PART_FILE))

spring-test/src/test/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilderTests.java

@@ -0,0 +1,57 @@
+/*
+ * Copyright 2002-2024 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.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.test.web.servlet.request;
+
+import jakarta.servlet.ServletContext;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.http.HttpMethod;
+import org.springframework.http.MediaType;
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockServletContext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link AbstractMockMultipartHttpServletRequestBuilder}.
+ *
+ * @author Stephane Nicoll
+ */
+public class AbstractMockMultipartHttpServletRequestBuilderTests {
+
+	private final ServletContext servletContext = new MockServletContext();
+
+
+	@Test
+	void builderSetsRequestContentType() {
+		MockHttpServletRequest request = buildRequest(new TestRequestBuilder(HttpMethod.PUT).uri("/upload"));
+		assertThat(request.getContentType()).isEqualTo(MediaType.MULTIPART_FORM_DATA_VALUE);
+	}
+
+
+	private MockHttpServletRequest buildRequest(AbstractMockHttpServletRequestBuilder<?> builder) {
+		return builder.buildRequest(this.servletContext);
+	}
+
+
+	private static class TestRequestBuilder extends AbstractMockMultipartHttpServletRequestBuilder<TestRequestBuilder> {
+
+		TestRequestBuilder(HttpMethod httpMethod) {
+			super(httpMethod);
+		}
+	}
+}