Currently with Salesforce Activity Sync jobs, the attachments of the synched Timeline entries are stored in Attachments object in Salesforce. This works well with classic implementation. Salesforce Lightning version doesnt support Attachments, it expects the attachments to be stored in Files.

Ex Ref: https://medium.com/@astrea_it/limitations-of-notes-and-attachments-in-salesforce-lightning-27a7a442cec8

So if a customer wants to move to Lightening, Timeline should be able to push attachments to Files instead of Attachments. Also it should be possible to move existing attachments to Files object.