Had the same problem but heading in the direction of a solution though not quite there yet. I include all my steps as it may help others.
Just upgraded from 1.14.1 to 1.14.2
osTicket Version v1.14.2 (cba6035) — Up to date
Web Server Software Apache
MySQL Version 5.6.47
PHP Version 7.3.14
CentOS Linux release 7.7.1908
(After install, Settings shows osTicket (v1.14.2) though there was no upgrade processing from 1.14.1. I assume that is OK).
I had included this plugin in the download and there was a surprisingly small extra file (storage-fs.phar) which I installed per instructions, set a Base Folder path (/sites/osticket/attachments) with folder attachments owned by apache, and enabled the plugin. All looks OK (The warning message when setting the path went away once I changed owner to apache)
I selected 'Filesystem' in Settings page for Attachments.
But when I added an attachment (using the Attachment tab in an FAQ) I could not see any file in the 'attachments' folder (other than zero length .keep)
I checked the php log immediately after the attach and there were no errors.
I then changed the ownership of the attachments folder from 775 to 777 and it worked. But that is really an issue as it makes the folder publicly accessible - so I put a dummy index.php in the folder as a work-round.
The next issue is that while deleting an attachment removes it from the document, it does not delete the saved file from the folder. Neither does it delete the MySQL record in ost_file.
These look like bugs but maybe I'm doing something wrong.