Copy headers from part in MultipartBodyBuilder

This commit makes sure that Part.headers() is copied over when adding a
part in the MultipartBodyBuilder.

Closes gh-26410

spring-web/src/main/java/org/springframework/http/client/MultipartBodyBuilder.java

@@ -30,7 +30,6 @@ import org.springframework.core.io.buffer.DataBuffer;
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
-import org.springframework.http.codec.multipart.FilePart;
 import org.springframework.http.codec.multipart.Part;
 import org.springframework.lang.NonNull;
 import org.springframework.lang.Nullable;
@@ -129,13 +128,14 @@ public final class MultipartBodyBuilder {
 		Assert.notNull(part, "'part' must not be null");
 
 		if (part instanceof Part) {
-			PartBuilder builder = asyncPart(name, ((Part) part).content(), DataBuffer.class);
+			Part partObject = (Part) part;
+			PartBuilder builder = asyncPart(name, partObject.content(), DataBuffer.class);
+			if (!partObject.headers().isEmpty()) {
+				builder.headers(headers -> headers.putAll(partObject.headers()));
+			}
 			if (contentType != null) {
 				builder.contentType(contentType);
 			}
-			if (part instanceof FilePart) {
-				builder.filename(((FilePart) part).filename());
-			}
 			return builder;
 		}
 

spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java

@@ -37,6 +37,7 @@ import org.springframework.core.io.buffer.DataBufferUtils;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.core.testfixture.io.buffer.AbstractLeakCheckingTests;
 import org.springframework.http.HttpEntity;
+import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.http.client.MultipartBodyBuilder;
 import org.springframework.http.codec.ClientCodecConfigurer;
@@ -102,8 +103,12 @@ public class MultipartHttpMessageWriterTests extends AbstractLeakCheckingTests {
 				this.bufferFactory.wrap("Cc".getBytes(StandardCharsets.UTF_8))
 		);
 		FilePart mockPart = mock(FilePart.class);
+		HttpHeaders partHeaders = new HttpHeaders();
+		partHeaders.setContentType(MediaType.TEXT_PLAIN);
+		partHeaders.setContentDispositionFormData("filePublisher", "file.txt");
+		partHeaders.add("foo", "bar");
+		given(mockPart.headers()).willReturn(partHeaders);
 		given(mockPart.content()).willReturn(bufferPublisher);
-		given(mockPart.filename()).willReturn("file.txt");
 
 		MultipartBodyBuilder bodyBuilder = new MultipartBodyBuilder();
 		bodyBuilder.part("name 1", "value 1");
@@ -166,6 +171,7 @@ public class MultipartHttpMessageWriterTests extends AbstractLeakCheckingTests {
 
 		part = requestParts.getFirst("filePublisher");
 		assertThat(part.name()).isEqualTo("filePublisher");
+		assertThat(part.headers()).containsEntry("foo", Collections.singletonList("bar"));
 		assertThat(((FilePart) part).filename()).isEqualTo("file.txt");
 		value = decodeToString(part);
 		assertThat(value).isEqualTo("AaBbCc");