[HTTP_Download] : Security and MIME types

Hi I use HTTP_Download apckage to set up a feature to download some files. Is reasonnable to set-up the 'contenttype' parameter in the staticSend function ? I mean to set tup this parameter (with Windows OS) I should guess it with the 3 last characters of the filename. But this extension can be changed by the users before uploading the file (the files to download can be upload by users in a previous process). Should I leave 'contenttype' to blank, just to open a "Save As" window with a "Save" option chosen by default? If I set a 'contenttype' based on the extension, this opens a "Save As" window with a Open option chosen by default. What do you think about that ? Does anybody have another way to guess the MIME type ? Mos