Recognize 7z files by magic bytes, not filename

Library/Homebrew/extend/pathname.rb

@@ -188,13 +188,13 @@ class Pathname
     when /^.{257}ustar/n        then :tar
     when /^\xFD7zXZ\x00/n       then :xz
     when /^Rar!/n               then :rar
+    when /^7z\xBC\xAF\x27\x1C/n then :p7zip
     else
       # This code so that bad-tarballs and zips produce good error messages
       # when they don't unarchive properly.
       case extname
       when ".tar.gz", ".tgz", ".tar.bz2", ".tbz" then :tar
       when ".zip" then :zip
-        when ".7z" then :p7zip
       end
     end
   end