The attachments are stored in the database or in the "Files" directory depending upon the setting at:
Admin CP > Settings > Server setting and performance > Storage location for file attachments.
To make more free space available, attachments in the tickets can be purged from: Admin CP > Maintenance > Purge Attachments. Further, attachments can be purged based on the criteria: File Name, File Size or Creation Date.
Note that 'Purge attachments' removes the attachments from Tickets as well from Knowledgebase articles.
If you wish to delete it only from Tickets, you can either upgrade your Kayako to latest version to select the section; or just let us know, we will share the source code to be tweaked.