Attachments can be removed from wiki pages or tickets under the
following circumstances:

*: The Setup user can delete an attachment at any time.

*: A registered user (a user other than anonymous) can delete an
   attachment that they themselves added within the previous 24 hours.

*: Users with the Delete privilege can delete attachments added
   by anonymous within the previous 24 hours.

When an attachment is deletable, a hyperlink called [Delete] appears
beside it in the attachment list at the bottom of the wiki page or
ticket screen.  Click on the hyperlink to delete the attachment.
There is no confirmation screen.  The deletion is permanent and cannot
be undone.

See also: DeleteTicket and DeleteWiki