MQFTE monitor to append the messages from queue to file

Question

I have a requirement where I want create different files from MQ messages based on the filename in the header. If a file is already existing in the folder location, the message in the queue should be appended to the contents of already existing file, else a new file should be created based on the filename in the header. I do not want to use File node because of some design constraint. I have tried creating MQFTE monitor to test this requirement but the result is: if a file is not existing, a new file is getting created however an existing file is getting overwritten with the new message instead of appending. What would be the best way to address this?