Validate attachment type in attachment data card while submitting the request

In my app , I want to restrict the attachment of types (.txt,.msg,.rtf). The users are allowed to add 6 attachments. Among these 6 attachments in that attachment data card how to identify the presence of restricted attachment.
I saw below code while searching
Last(!Split(Last(data card.attachments).Name,".")).Result in ("txt")
But this on will validate only the last file among the 6 attachments in the data card.
Kindly help me to solve this issue.