Go into Admin Panel - Settings - Attachments
Check "Allow Attachments"
Check "Accept Email Attachments"
Check "Allow online attachments upload"
Leave "Authenticated users Only. (User must be logged in to upload files)" unchecked if you want anyone to be albe to upload an attachment. Check it if you only want this option available to users who are already logged in/are respodning to currently open tickets.
Enter the maximum filesize in bytes (or just use the system default)
In "Attachment folder" enter the full path to where you want attachments stored. This directory must be writable (CHMOD to 0755). Please note that this is the absolute path to the folder on your server, NOT a URL ('/home/myusername/www/osticket/attachments/' for example). For added security, you can place this folder outside of the web root if desired.
Under "Accepted File Types" enter a list of acceptable file extensions, separated by a comma (e.g. '.txt, .doc, .jpg')