try this
ROBOCOPY C:\inetpub D:\inetpub-backups\Friday %_what% %_options% /XD *file_hosting /XF " ar.*" /LOG:"%RCLogFile%"
..or this:
ROBOCOPY C:\inetpub D:\inetpub-backups\Friday %_what% %_options% /XD *file_hosting /XF " *ar.*" /LOG:"%RCLogFile%"
If the search pattern contains spaces, it must be enclosed in double quotes.