# CJam, 18 bytes

    ea_:gs{q}?'_,_eler

The list of files must be supplied in form of URLs, which is the only format CJam understands.

### Example runs

    $ cjam kitten <<< "AaÁáÄä"
    aaÁáÄä
    $ cjam kitten file:///home/dennis/kitten file:///home/dennis/kitten
    ea_:gs{q}?'_,_elerea_:gs{q}?'_,_eler

### How it works

    ea                  Push the array of command-line arguments.
      _                 Push a copy.
       :g               Retrieve the contents of all files with those URLS.
         s              Flatten the resulting array of strings.
          {q}           Push a block that reads all input from STDIN.
             ?          Select the string of the array of args is non-empty.
                        Otherwise, execute the code block.
              '_,       Push the string of all ASCII characters before _.
                 _el    Push a copy and convert it to lowercase.
                    er  Perform transliteration.