There are settings in backend of osticket just enable those ones
step1- login to backend of osticket with admin privileges
step2- click on tab settings(next to dashboard).
step3- within settings click on tab tickets.
step4- There is Attachment section on this page.enable checkbox in front of
**Allow Attachments,online web attachments** etc.
step5- change other settings as per requirement also you can specify the required file types like .jpg,.png along with .doc and .pdf.
step6- Save the changes.
If you want more customization you can edit file here osticket/include/client/open.inc.php