Hi Mohit,
Additional Files always enter PI as attachments. So if you are planning that Single Sender File Adapter will create separate messages for '.xml' and 'abc.txt', that would not happen if both of them are present at same polling time. It would be a single message with attachment.
Again if you have to apply extra configurations to write them as separate files.
Specifying Additional Files in the Sender File Adapter - Advanced Adapter Engine - SAP Library
"Use In the sender file adapter, you can specify additional files to be included in the XI message as attachments. The receiver of such messages must be able to process them. Additional files are transported unchanged without conversion options."
Regards,
Nabendu.