List of file name suffixes and associated decoding commands.
Each entry should be (SUFFIX . STRING); the file is given to
the command as standard input.
STRING may be a list of strings. In that case, the first element is
the command name, and the rest are arguments to that command.
If STRING is nil, no decoding is done.
Because the SUFFIXes are tried in order, the empty string should be last in the list.