Support XLS/XLSX attachments on Twilio WhatsApp channels

Problem
Front blocks XLS and XLSX file attachments on Twilio WhatsApp channels even though Twilio's WhatsApp API natively supports these file types. Users must convert spreadsheets to PDF or switch to email to share Excel files with contacts.

Request
Enable sending and receiving XLS and XLSX file attachments through the Twilio WhatsApp channel in Front.