Fix canWrite of PartHttpMessageWriter

See gh-29631

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

@@ -48,6 +48,21 @@ public class PartHttpMessageWriter extends MultipartWriterSupport implements Htt
 		super(MultipartHttpMessageReader.MIME_TYPES);
 	}
 
+	@Override
+	public boolean canWrite(ResolvableType elementType, @Nullable MediaType mediaType) {
+		if (Part.class.isAssignableFrom(elementType.toClass())) {
+			if (mediaType == null) {
+				return true;
+			}
+			for (MediaType supportedMediaType : getWritableMediaTypes()) {
+				if (supportedMediaType.isCompatibleWith(mediaType)) {
+					return true;
+				}
+			}
+		}
+		return false;
+	}
+
 
 	@Override
 	public Mono<Void> write(Publisher<? extends Part> parts,