diff --git a/spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java b/spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java
index 041cf24f6e10..8c9a4f9fb7ac 100644
--- a/spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java
+++ b/spring-test/src/main/java/org/springframework/test/web/servlet/TestDispatcherServlet.java
@@ -35,7 +35,6 @@
 import org.springframework.web.context.request.async.DeferredResult;
 import org.springframework.web.context.request.async.DeferredResultProcessingInterceptor;
 import org.springframework.web.context.request.async.WebAsyncUtils;
-import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
 import org.springframework.web.servlet.DispatcherServlet;
 import org.springframework.web.servlet.HandlerExecutionChain;
 import org.springframework.web.servlet.ModelAndView;
@@ -68,10 +67,6 @@ public TestDispatcherServlet(WebApplicationContext webApplicationContext) {
 	protected void service(HttpServletRequest request, HttpServletResponse response)
 			throws ServletException, IOException {
 
-		if (!request.getParts().isEmpty()) {
-			request = new StandardMultipartHttpServletRequest(request);
-		}
-
 		registerAsyncResultInterceptors(request);
 
 		super.service(request, response);
diff --git a/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilder.java b/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilder.java
index 75cb972331be..c0f15fcc81c8 100644
--- a/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilder.java
+++ b/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilder.java
@@ -17,7 +17,10 @@
 package org.springframework.test.web.servlet.request;
 
 import java.io.IOException;
+import java.io.InputStreamReader;
 import java.net.URI;
+import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.List;
@@ -25,17 +28,17 @@
 import javax.servlet.ServletContext;
 import javax.servlet.http.Part;
 
-import org.springframework.http.HttpHeaders;
 import org.springframework.http.HttpMethod;
 import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
 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.util.Assert;
+import org.springframework.util.FileCopyUtils;
 import org.springframework.util.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
+import org.springframework.web.multipart.MultipartFile;
 
 /**
  * Default builder for {@link MockMultipartHttpServletRequest}.
@@ -141,26 +144,47 @@ public Object merge(@Nullable Object parent) {
 	@Override
 	protected final MockHttpServletRequest createServletRequest(ServletContext servletContext) {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest(servletContext);
-		this.files.forEach(file -> request.addPart(toMockPart(file)));
-		this.parts.values().stream().flatMap(Collection::stream).forEach(request::addPart);
-		return request;
-	}
-
-	private MockPart toMockPart(MockMultipartFile file) {
-		byte[] bytes = null;
-		if (!file.isEmpty()) {
+		this.files.forEach(request::addFile);
+		this.parts.values().stream().flatMap(Collection::stream).forEach(part -> {
+			request.addPart(part);
 			try {
-				bytes = file.getBytes();
+				MultipartFile file = asMultipartFile(part);
+				if (file != null) {
+					request.addFile(file);
+					return;
+				}
+				String value = toParameterValue(part);
+				if (value != null) {
+					request.addParameter(part.getName(), toParameterValue(part));
+				}
 			}
 			catch (IOException ex) {
-				throw new IllegalStateException("Unexpected IOException", ex);
+				throw new IllegalStateException("Failed to read content for part " + part.getName(), ex);
 			}
+		});
+		return request;
+	}
+
+	@Nullable
+	private MultipartFile asMultipartFile(Part part) throws IOException {
+		String name = part.getName();
+		String filename = part.getSubmittedFileName();
+		if (filename != null) {
+			return new MockMultipartFile(name, filename, part.getContentType(), part.getInputStream());
 		}
-		MockPart part = new MockPart(file.getName(), file.getOriginalFilename(), bytes);
-		if (file.getContentType() != null) {
-			part.getHeaders().set(HttpHeaders.CONTENT_TYPE, file.getContentType());
+		return null;
+	}
+
+	@Nullable
+	private String toParameterValue(Part part) throws IOException {
+		String rawType = part.getContentType();
+		MediaType mediaType = (rawType != null ? MediaType.parseMediaType(rawType) : MediaType.TEXT_PLAIN);
+		if (!mediaType.isCompatibleWith(MediaType.TEXT_PLAIN)) {
+			return null;
 		}
-		return part;
+		Charset charset = (mediaType.getCharset() != null ? mediaType.getCharset() : StandardCharsets.UTF_8);
+		InputStreamReader reader = new InputStreamReader(part.getInputStream(), charset);
+		return FileCopyUtils.copyToString(reader);
 	}
 
 }
diff --git a/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilderTests.java b/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilderTests.java
index a7087f424fb0..b1414d2bf4c2 100644
--- a/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilderTests.java
+++ b/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockMultipartHttpServletRequestBuilderTests.java
@@ -16,19 +16,19 @@
 
 package org.springframework.test.web.servlet.request;
 
-import java.nio.charset.StandardCharsets;
-
 import javax.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 org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
 
+import static java.nio.charset.StandardCharsets.UTF_8;
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
@@ -38,17 +38,32 @@
 public class MockMultipartHttpServletRequestBuilderTests {
 
 	@Test // gh-26166
-	void addFilesAndParts() throws Exception {
-		MockHttpServletRequest mockRequest = new MockMultipartHttpServletRequestBuilder("/upload")
-				.file(new MockMultipartFile("file", "test.txt", "text/plain", "Test".getBytes(StandardCharsets.UTF_8)))
-				.part(new MockPart("data", "{\"node\":\"node\"}".getBytes(StandardCharsets.UTF_8)))
-				.buildRequest(new MockServletContext());
+	void addFileAndParts() throws Exception {
+		MockMultipartHttpServletRequest mockRequest =
+				(MockMultipartHttpServletRequest) new MockMultipartHttpServletRequestBuilder("/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
+	void addFileWithoutFilename() throws Exception {
+		MockPart jsonPart = new MockPart("data", "{\"node\":\"node\"}".getBytes(UTF_8));
+		jsonPart.getHeaders().setContentType(MediaType.APPLICATION_JSON);
 
-		StandardMultipartHttpServletRequest parsedRequest = new StandardMultipartHttpServletRequest(mockRequest);
+		MockMultipartHttpServletRequest mockRequest =
+				(MockMultipartHttpServletRequest) new MockMultipartHttpServletRequestBuilder("/upload")
+						.file(new MockMultipartFile("file", "Test".getBytes(UTF_8)))
+						.part(jsonPart)
+						.buildRequest(new MockServletContext());
 
-		assertThat(parsedRequest.getParameterMap()).containsOnlyKeys("data");
-		assertThat(parsedRequest.getFileMap()).containsOnlyKeys("file");
-		assertThat(parsedRequest.getParts()).extracting(Part::getName).containsExactly("file", "data");
+		assertThat(mockRequest.getFileMap()).containsOnlyKeys("file");
+		assertThat(mockRequest.getParameterMap()).isEmpty();
+		assertThat(mockRequest.getParts()).extracting(Part::getName).containsExactly("data");
 	}
 
 	@Test