Alphanumeric tools::file_path_sans_ext() (PR#14050)

The file_path_sans_ext() function in the 'tools' package does not handle 
alphanumeric file extensions correctly:

   require(tools)
   file_path_sans_ext("song.txt")  # song, correct
   file_path_sans_ext("song.mp3")  # song.mp3, wrong

The help page states that "only purely alphanumeric extensions are 
recognized", which I had expected. To fulfill this, the function body 
should be

   sub("([^.]+)\\.[[:alnum:]]+$", "\\1", x)

instead of the current definition:

   sub("([^.]+)\\.[[:alpha:]]+$", "\\1", x)

Thanks,

Arni