IMHO, no need to add such feature since osTicket has provided two very simple upload feature from the two sides. One from client-side, and the other from staff-side. From the client-side you may use open and post reply ticket form, and from the staff-side you may also use create and post reply ticket form.
Yes, there is a mod somewhere in this forum that provides upload from Internal Notes section. Of course, the upload feature in Internal Notes section should be provided only for internal staff purpose (not for client).
If you implement another way besides the two main ways above, then actually you will duplicate the existing upload feature from staff-side (just my two cents).
However, if you meant the quick and simple way to view the list of the attachment files, then consider to implement the attachments list mod via (this link).
Hope it helps.
Sincerely,
Masino Sinaga