Files are uploaded as Salesforce Files and not as Attachments

Notes:

• This feature only applies to upload of documents in Classic user interface via the Notes & Attachments related list.  
• In Lightning regardless of whether the feature is on or off, uploading a document in the Notes and Attachments related list will still result in the document being added as a Salesforce File.  
• This feature will not convert email attachments in the Salesforce Classic UI. Email attachments will continue to be Attachments and not converted to Files.  
• This feature does not work if you upload documents or Attachment records using Data Loader or other API tools. The tool will import the document on the specific object, either Files (ContentVersion) or Attachments; you selected at the start of the insert process.
• Enabling this feature does not retroactively convert existing Attachment records and only applies to newly added files.

It's unlikely there will be any additional changes to this in the future as all new Salesforce innovation is being focused in Lightning Experience. We encourage you to transition fully to Lightning Experience and start benefiting from everything it has to offer.