settin mine type while saving files

Hi there, I am creating pdf files inside my application and do save them to the server file system. Now I have sent some of the emails by the system via email and found that the mine type is not set. So the pdf file apears in the attachement, but one has to select the application from a list to display it. Is there a way to save the mime type with the file, so windows does know how to handle that file? Thanx for any help on that, Merlin