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 c0f15fcc81c..73a0a3bd29d 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
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -17,6 +17,7 @@
 package org.springframework.test.web.servlet.request;
 
 import java.io.IOException;
+import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.net.URI;
 import java.nio.charset.Charset;
@@ -38,7 +39,6 @@ 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}.
@@ -144,47 +144,40 @@ public class MockMultipartHttpServletRequestBuilder extends MockHttpServletReque
 	@Override
 	protected final MockHttpServletRequest createServletRequest(ServletContext servletContext) {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest(servletContext);
+		Charset defaultCharset = (request.getCharacterEncoding() != null ?
+				Charset.forName(request.getCharacterEncoding()) : StandardCharsets.UTF_8);
+
 		this.files.forEach(request::addFile);
 		this.parts.values().stream().flatMap(Collection::stream).forEach(part -> {
 			request.addPart(part);
 			try {
-				MultipartFile file = asMultipartFile(part);
-				if (file != null) {
-					request.addFile(file);
-					return;
+				String name = part.getName();
+				String filename = part.getSubmittedFileName();
+				InputStream is = part.getInputStream();
+				if (filename != null) {
+					request.addFile(new MockMultipartFile(name, filename, part.getContentType(), is));
 				}
-				String value = toParameterValue(part);
-				if (value != null) {
-					request.addParameter(part.getName(), toParameterValue(part));
+				else {
+					InputStreamReader reader = new InputStreamReader(is, getCharsetOrDefault(part, defaultCharset));
+					String value = FileCopyUtils.copyToString(reader);
+					request.addParameter(part.getName(), value);
 				}
 			}
 			catch (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());
+	private Charset getCharsetOrDefault(Part part, Charset defaultCharset) {
+		if (part.getContentType() != null) {
+			MediaType mediaType = MediaType.parseMediaType(part.getContentType());
+			if (mediaType.getCharset() != null) {
+				return mediaType.getCharset();
+			}
 		}
-		return null;
+		return defaultCharset;
 	}
-
-	@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;
-		}
-		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 b1414d2bf4c..22263b90959 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
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 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.
@@ -50,7 +50,7 @@ public class MockMultipartHttpServletRequestBuilderTests {
 		assertThat(mockRequest.getParts()).extracting(Part::getName).containsExactly("name");
 	}
 
-	@Test // gh-26261
+	@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);
@@ -62,7 +62,8 @@ public class MockMultipartHttpServletRequestBuilderTests {
 						.buildRequest(new MockServletContext());
 
 		assertThat(mockRequest.getFileMap()).containsOnlyKeys("file");
-		assertThat(mockRequest.getParameterMap()).isEmpty();
+		assertThat(mockRequest.getParameterMap()).hasSize(1);
+		assertThat(mockRequest.getParameter("data")).isEqualTo("{\"node\":\"node\"}");
 		assertThat(mockRequest.getParts()).extracting(Part::getName).containsExactly("data");
 	}