diff --git a/spring-test/src/test/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilderTests.java b/spring-test/src/test/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilderTests.java
index ea4b4a95b8..0ae56091fb 100644
--- a/spring-test/src/test/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilderTests.java
+++ b/spring-test/src/test/java/org/springframework/test/web/servlet/request/AbstractMockMultipartHttpServletRequestBuilderTests.java
@@ -17,13 +17,18 @@
 package org.springframework.test.web.servlet.request;
 
 import jakarta.servlet.ServletContext;
+import jakarta.servlet.http.Part;
 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.MockMultipartFile;
+import org.springframework.mock.web.MockMultipartHttpServletRequest;
+import org.springframework.mock.web.MockPart;
 import org.springframework.mock.web.MockServletContext;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
@@ -35,13 +40,60 @@ public class AbstractMockMultipartHttpServletRequestBuilderTests {
 
 	private final ServletContext servletContext = new MockServletContext();
 
+	@Test // gh-26166
+	void addFileAndParts() throws Exception {
+		MockMultipartHttpServletRequest mockRequest =
+				(MockMultipartHttpServletRequest) createBuilder("/upload")
+						.file(new MockMultipartFile("file", "test.txt", "text/plain", "Test".getBytes(UTF_8)))
+						.part(new MockPart("name", "value".getBytes(UTF_8)))
+						.buildRequest(new MockServletContext());
+
+		assertThat(mockRequest.getFileMap()).containsOnlyKeys("file");
+		assertThat(mockRequest.getParameterMap()).containsOnlyKeys("name");
+		assertThat(mockRequest.getParts()).extracting(Part::getName).containsExactly("name");
+	}
+
+	@Test // gh-26261, gh-26400
+	void addFileWithoutFilename() throws Exception {
+		MockPart jsonPart = new MockPart("data", "{\"node\":\"node\"}".getBytes(UTF_8));
+		jsonPart.getHeaders().setContentType(MediaType.APPLICATION_JSON);
+
+		MockMultipartHttpServletRequest mockRequest =
+				(MockMultipartHttpServletRequest) createBuilder("/upload")
+						.file(new MockMultipartFile("file", "Test".getBytes(UTF_8)))
+						.part(jsonPart)
+						.buildRequest(new MockServletContext());
+
+		assertThat(mockRequest.getFileMap()).containsOnlyKeys("file");
+		assertThat(mockRequest.getParameterMap()).hasSize(1);
+		assertThat(mockRequest.getParameter("data")).isEqualTo("{\"node\":\"node\"}");
+		assertThat(mockRequest.getParts()).extracting(Part::getName).containsExactly("data");
+	}
+
+	@Test
+	void mergeAndBuild() {
+		MockHttpServletRequestBuilder parent = new MockHttpServletRequestBuilder(HttpMethod.GET).uri("/");
+		parent.characterEncoding("UTF-8");
+		Object result = createBuilder("/fileUpload").merge(parent);
+
+		assertThat(result).isNotNull();
+		assertThat(result.getClass()).isEqualTo(TestRequestBuilder.class);
+
+		TestRequestBuilder builder = (TestRequestBuilder) result;
+		MockHttpServletRequest request = builder.buildRequest(new MockServletContext());
+		assertThat(request.getCharacterEncoding()).isEqualTo("UTF-8");
+	}
+
 
 	@Test
 	void builderSetsRequestContentType() {
-		MockHttpServletRequest request = buildRequest(new TestRequestBuilder(HttpMethod.PUT).uri("/upload"));
+		MockHttpServletRequest request = buildRequest(createBuilder("/upload"));
 		assertThat(request.getContentType()).isEqualTo(MediaType.MULTIPART_FORM_DATA_VALUE);
 	}
 
+	private TestRequestBuilder createBuilder(String uri) {
+		return new TestRequestBuilder(HttpMethod.POST).uri(uri);
+	}
 
 	private MockHttpServletRequest buildRequest(AbstractMockHttpServletRequestBuilder<?> builder) {
 		return builder.buildRequest(this.servletContext);