Attachment is being deprecated in Salesforce Lightning Experience. Attachments from Salesforce Classic won’t be available in Salesforce Lightning Experience Search Result.
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To automatically convert files attached in Salesforce Classic as Files and not as Attachments, enable “Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments”. This is available in Salesforce Setup –> Customize –> Salesforce Files –> Settings –> General Settings.
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To migrate Notes and Attachments from Classic to Lightning, use Magic Mover.
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