add ar archive type

even though it is already in standard file magic

but ar archives could be detected as text/plain otherwise because the files are not detected as application/octet-stream

fact_helper_file/mime/custom_mime_archives

@@ -137,3 +137,9 @@
 # sharutils shell archive
 12   string  This\x20is\x20a\x20shell\x20archive    sharutils shell archive
 !:mime	application/x-shar
+
+# ar archive
+0	string		=!<arch>\n		current ar archive
+!:mime	application/x-archive
+0	string		=<ar>		System V Release 1 ar archive
+!:mime	application/x-archive