When this feature is enabled, users can choose to upload a file as an ATTACHMENT. By default, files are uploaded as Content. This feature allows users to decide whether to upload a file as Content or as an ATTACHMENT.
If you would like to learn more about this feature, please visit the following link: Mass File Upload: Additional Feature - Attachment Support
Demo Video: https://www.youtube.com/watch?v=vfKJGJ9fYbQ&list=PLXqeMW5vDpMA_VqPcBieEV7ZvDgkVYVGv&index=2
PS: View Mass File Upload, Salesforce file upload app on Salesforce AppExchange: https://appexchange.salesforce.com/appxListingDetail?listingId=82cf236b-1ee8-4ea7-aebf-fd0c89fb93bd