Ensure filename is written

Now that SPR-16307 makes it possible to write a part with a Publisher
we need to ensure we get the filename correctly when writing with
Publisher<Resource>.

Issue: SPR-16376

Author: rstoyanchev

spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java

@@ -252,8 +252,17 @@ private <T> Flux<DataBuffer> encodePart(byte[] boundary, String name, T value) {
 			resolvableType = ResolvableType.forClass(body.getClass());
 		}
 
-		String filename = (body instanceof Resource ? ((Resource) body).getFilename() : null);
-		outputMessage.getHeaders().setContentDispositionFormData(name, filename);
+		if (body instanceof Resource) {
+			outputMessage.getHeaders().setContentDispositionFormData(name, ((Resource) body).getFilename());
+		}
+		else if (Resource.class.equals(resolvableType.getRawClass())) {
+			body = (T) Mono.from((Publisher<?>) body).doOnNext(o -> {
+				outputMessage.getHeaders().setContentDispositionFormData(name, ((Resource) o).getFilename());
+			});
+		}
+		else {
+			outputMessage.getHeaders().setContentDispositionFormData(name, null);
+		}
 
 		MediaType contentType = outputMessage.getHeaders().getContentType();
 

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

@@ -170,9 +170,8 @@ public void singleSubscriberWithResource() throws IOException {
 
 		Part part = requestParts.getFirst("logo");
 		assertEquals("logo", part.name());
-// TODO: a Resource written as an async part doesn't have a file name in the contentDisposition
-//		assertTrue(part instanceof FilePart);
-//		assertEquals("logo.jpg", ((FilePart) part).filename());
+		assertTrue(part instanceof FilePart);
+		assertEquals("logo.jpg", ((FilePart) part).filename());
 		assertEquals(MediaType.IMAGE_JPEG, part.headers().getContentType());
 		assertEquals(logo.getFile().length(), part.headers().getContentLength());
 	}