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 dea33261c2..75cb972331 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
@@ -16,6 +16,7 @@
 
 package org.springframework.test.web.servlet.request;
 
+import java.io.IOException;
 import java.net.URI;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -24,12 +25,14 @@ import java.util.List;
 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.LinkedMultiValueMap;
 import org.springframework.util.MultiValueMap;
@@ -119,7 +122,7 @@ public class MockMultipartHttpServletRequestBuilder extends MockHttpServletReque
 			if (parent instanceof MockMultipartHttpServletRequestBuilder) {
 				MockMultipartHttpServletRequestBuilder parentBuilder = (MockMultipartHttpServletRequestBuilder) parent;
 				this.files.addAll(parentBuilder.files);
-				parentBuilder.parts.keySet().stream().forEach(name ->
+				parentBuilder.parts.keySet().forEach(name ->
 						this.parts.putIfAbsent(name, parentBuilder.parts.get(name)));
 			}
 
@@ -138,9 +141,26 @@ public class MockMultipartHttpServletRequestBuilder extends MockHttpServletReque
 	@Override
 	protected final MockHttpServletRequest createServletRequest(ServletContext servletContext) {
 		MockMultipartHttpServletRequest request = new MockMultipartHttpServletRequest(servletContext);
-		this.files.forEach(request::addFile);
+		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()) {
+			try {
+				bytes = file.getBytes();
+			}
+			catch (IOException ex) {
+				throw new IllegalStateException("Unexpected IOException", ex);
+			}
+		}
+		MockPart part = new MockPart(file.getName(), file.getOriginalFilename(), bytes);
+		if (file.getContentType() != null) {
+			part.getHeaders().set(HttpHeaders.CONTENT_TYPE, file.getContentType());
+		}
+		return part;
+	}
+
 }
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 82f039d590..a7087f424f 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 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -16,21 +16,43 @@
 
 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.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockMultipartFile;
+import org.springframework.mock.web.MockPart;
 import org.springframework.mock.web.MockServletContext;
+import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
 
 import static org.assertj.core.api.Assertions.assertThat;
 
 /**
+ * Unit tests for {@link MockMultipartHttpServletRequestBuilder}.
  * @author Rossen Stoyanchev
  */
 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());
+
+		StandardMultipartHttpServletRequest parsedRequest = new StandardMultipartHttpServletRequest(mockRequest);
+
+		assertThat(parsedRequest.getParameterMap()).containsOnlyKeys("data");
+		assertThat(parsedRequest.getFileMap()).containsOnlyKeys("file");
+		assertThat(parsedRequest.getParts()).extracting(Part::getName).containsExactly("file", "data");
+	}
+
 	@Test
-	public void test() {
+	void mergeAndBuild() {
 		MockHttpServletRequestBuilder parent = new MockHttpServletRequestBuilder(HttpMethod.GET, "/");
 		parent.characterEncoding("UTF-8");
 		Object result = new MockMultipartHttpServletRequestBuilder("/fileUpload").merge(parent);