Download page Migrating from Notes and Attachments to Files in Lightning.
Migrating from Notes and Attachments to Files in Lightning
When you switch to Lightning, any files you upload to a record will be saved as Salesforce Files, rather than as attachments.
To ensure that your org is using Files, enable the following settings:
Go to Setup > General Settings. Select Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments.
Go to Setup > Custom Settings > Comply System Properties. Select Enable Files.
Please note the following:
After you enable Files, uploads are stored on both the "Files" and "Notes And Attachments" related lists.
The "Files uploaded to the Attachments related list on records are uploaded as Salesforce Files, not as attachments" setting causes files that are uploaded to be stored as the "File" type rather than the "Attachment" type.