diff --git a/spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilder.java b/spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilder.java
index fa52c987c667..7d76e63e7511 100644
--- a/spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilder.java
+++ b/spring-test/src/main/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilder.java
@@ -377,12 +377,18 @@ private void params(MockHttpServletRequest request, UriComponents uriComponents)
 				MockPart part;
 				if (file != null) {
 					part = new MockPart(pair.getName(), file.getName(), readAllBytes(file));
-					part.getHeaders().setContentType(MediaType.valueOf(pair.getMimeType()));
 				}
-				else { // mimic empty file upload
-					part = new MockPart(pair.getName(), "", null);
-					part.getHeaders().setContentType(MediaType.APPLICATION_OCTET_STREAM);
+				else {
+					// Support empty file upload OR file upload via setData().
+					// For an empty file upload, getValue() returns an empty string, and
+					// getData() returns null.
+					// For a file upload via setData(), getData() returns the file data, and
+					// getValue() returns the file name (if set) or an empty string.
+					part = new MockPart(pair.getName(), pair.getValue(), pair.getData());
 				}
+				MediaType mediaType = (pair.getMimeType() != null ? MediaType.valueOf(pair.getMimeType()) :
+						MediaType.APPLICATION_OCTET_STREAM);
+				part.getHeaders().setContentType(mediaType);
 				request.addPart(part);
 			}
 			else {
diff --git a/spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java b/spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java
index b1f73b4741f9..96ebbb4ed529 100644
--- a/spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java
+++ b/spring-test/src/test/java/org/springframework/test/web/servlet/htmlunit/HtmlUnitRequestBuilderTests.java
@@ -16,6 +16,7 @@
 
 package org.springframework.test.web.servlet.htmlunit;
 
+import java.io.File;
 import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URL;
@@ -443,6 +444,33 @@ public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithFile
 		assertThat(part.getContentType()).isEqualTo(MimeType.TEXT_PLAIN);
 	}
 
+	@Test // gh-27199
+	public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithFileDataAsParameter() throws Exception {
+		String data = "{}";
+		KeyDataPair keyDataPair = new KeyDataPair("key", new File("test.json"), null, MimeType.APPLICATION_JSON, StandardCharsets.UTF_8);
+		keyDataPair.setData(data.getBytes());
+
+		webRequest.setRequestParameters(Collections.singletonList(keyDataPair));
+
+		MockHttpServletRequest actualRequest = requestBuilder.buildRequest(servletContext);
+
+		assertThat(actualRequest.getParts()).hasSize(1);
+		Part part = actualRequest.getPart("key");
+
+		assertSoftly(softly -> {
+			softly.assertThat(part).as("part").isNotNull();
+			softly.assertThat(part.getName()).as("name").isEqualTo("key");
+			softly.assertThat(part.getSubmittedFileName()).as("file name").isEqualTo("test.json");
+			softly.assertThat(part.getContentType()).as("content type").isEqualTo(MimeType.APPLICATION_JSON);
+			try {
+				softly.assertThat(IOUtils.toString(part.getInputStream(), StandardCharsets.UTF_8)).as("content").isEqualTo(data);
+			}
+			catch (IOException ex) {
+				softly.fail("failed to get InputStream", ex);
+			}
+		});
+	}
+
 	@Test // gh-26799
 	public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithNullFileToUploadAsParameter() throws Exception {
 		webRequest.setRequestParameters(Collections.singletonList(new KeyDataPair("key", null, null, null, (Charset) null)));
@@ -453,11 +481,11 @@ public void buildRequestParameterMapViaWebRequestDotSetRequestParametersWithNull
 		Part part = actualRequest.getPart("key");
 
 		assertSoftly(softly -> {
-			softly.assertThat(part).isNotNull();
+			softly.assertThat(part).as("part").isNotNull();
 			softly.assertThat(part.getName()).as("name").isEqualTo("key");
 			softly.assertThat(part.getSize()).as("size").isEqualTo(0);
 			try {
-				softly.assertThat(part.getInputStream()).isEmpty();
+				softly.assertThat(part.getInputStream()).as("input stream").isEmpty();
 			}
 			catch (IOException ex) {
 				softly.fail("failed to get InputStream", ex);