Add back the logic to clear the files after use in MessageAction

src/main/java/net/dv8tion/jda/internal/requests/restaction/MessageActionImpl.java

@@ -528,6 +528,8 @@ protected RequestBody asMultipart()
     {
         MultipartBody.Builder body = AttachedFile.createMultipartBody(files, null);
         body.addFormDataPart("payload_json", getJSON().toString());
+        // clear remaining resources, they will be closed after being sent
+        files.clear();
         return body.build();
     }