Make RAR/RAR5 detection more robust.

sabnzbd/utils/rarfile.py

@@ -109,8 +109,8 @@ RAR_OS_UNIX  = 3
 def is_rarfile(fn):
     '''Check quickly whether file is rar archive.'''
     try:
-        buf = open(fn, "rb").read(len(RAR_ID))
-        return buf == RAR_ID or buf == RAR5_ID
+        buf = open(fn, "rb").read(50)
+        return buf.startswith(RAR_ID) or buf.startswith(RAR5_ID)
     except:
         return False