files: Only allow files uploaded in this session

This fixes a security issue where, by crafting a special POST request to the
client open.php page, an (unauthenticated) user could get a URL link to
access to any attachment already uploaded in the system by guessing or
brute-forcing the file's ID number.

This patch addresses the issue by registering the uploaded file's ID in the
current user's session. When processing the list of file ID's attached to
the FileUploadField, the files must already have been attached to the field
or have been newly attached in the current session.

Fixes #2615

References:
"Security issue - Download attachments submitted by others"
https://github.com/osTicket/osTicket-1.8/issues/2615