Attachments are saved in the file system on the server or in any network location that is visible to the application. The location can be configured in the Settings -> General -> Attachments Folder.
By default this location is C:\ProgramData\BSC\Files. If you look at this location, the ticket related attachments are stored in subfolders [Attachment Folder]\Ticket[ticket-friendly-number].
The ticket-friendly-number is a derived value of ticket type and ticket number. For example, #IN-13196 is the value for ticket number 13196 of ticket type Incident which has a mnemonic of #IN.
If you changed the ticket type of a ticket say from Incident to Service Request, the application will look for a ticket-friendly-number of #SR-13196 and the old attachment links will not work.
work around: copy the files to the expected location.