File types: optimized regex, updated list

Examined 5000 files downloaded our of 9964 files in 14 month span (98%
of total events). New order: pdf, docx?, xlsx?, pptx?, zip, mp\\d, rtf,
txt. Added exe (Windows executable), app (Mac executable) and apk
(Android apps). Removed rar. Consider removing rtf (0.2% of events) and
txt (0.04% of events).