I have a custom object and I want to add a new field to support file attachments. For example, while creating new record, I want the user/admin to have an option to attach files to that record during creation. I also have an active process that will automatically sent email with the data to the user. I want to sent that attachment/file as well in the email.
You cannot attach a file to a custom object record at the time of creation of that record. There is no custom field type that accepts file uploads.
You will need to restructure your business flow to have the record be created, use out-of-the-box functionality (the Files related list) to attach a file, and then build automation that does not fire on record creation to construct and send your email, perhaps triggered by a status update of some kind on the parent record.