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At the moment, it is not possible to use files attached to an SObject under the Notes and Attachment (through the Attachment API) in the LEX Email composer.
The need to download these files and re-upload them as Files to send them is at least surprising. Are Files are an alternative to Notes and Attachment, should they be preferred in some ways, and why such a limitation hold?
'Files' are the way to go now in Lightning Experience. Ideally, your users whether they are in classic or in Lightning should use 'Files' related list to upload files. You can see the difference between Attachments and File in the below link https://help.salesforce.com/articleView?id=lex_gaps_limitations_files_and_content.htm&type=5
https://help.salesforce.com/articleView?id=collab_files_differences.htm&type=5 https://help.salesforce.com/articleView?id=000337432&type=1&mode=1
Also, you can refer the below link for more info Difference between File and Attachment in SalesForce
