When a CI gets reclassified, the attachments on the CI form will not get carried to the new class. For example, if the CI currently belongs to a Windows Server class and when you run discovery, it classifies it as a computer. In that case, the attachments that used to be part of the form when the record belonged to windows server class, will not be present in the CI form, when it belongs to computer class.
Whenever you add an attachment to a form, the attachment is stored in sys_attachment table. The way it correlates to the form is by having two reference fields "Table name" and "Table sys ID"
So when a CI gets re-classified, the sys _id remains the same but the table name changes. So the attachment will not be able to correlate to the CI once the class/table is changed. You would need to manually change the entry on "Table name" field in the sys_attachment record, to point it to the correct CI