string match simplification?

[Ray Andrews <rayandrews@xxxxxxxxxxx>]

    I'm wanting to cut filenames at the
      extension, if any, but only if it's a 'real' extension.  Dunno if
      there's any formality to the idea but whereas you can put a period
      anywhere in a filename, we do recognize 'real' extensions like
      '.eml', '.html' '.png' ... and so on.  Seems to me that real
      extensions are almost never longer than four characters, say five
      to be safe.  Anyway the code below seems fine, but it bothers me
      that if there's no extension at all the variable 'extension' gets
      set to the entire string whereas I'd expect if there's no match it
      would return null.  Why return an obviously incorrect answer?  So
      there's that line to kill $extension if no match.  It's a trivial
      issue but is there a less belabored way?  
      
      #!/usr/bin/zsh
      
      filename=$1
      extension="${filename##*.}"
      (( $#filename == $#extension )) && extension=
      if (( $#extension > 0 && $#extension < 6 )); then
          filename="${filename%.*}"
      else
          extension=
      fi                                            
      echo filename is: $filename
      echo extension is: $extension