return response()->json(json_decode(file_get_contents('some/path', true)));

{
  "ID": "123",
  "Date": "08/06/2012",
  "Amount": "�120.59"
}

$data = file_get_contents('some/path', true);
$encoding = mb_detect_encoding($data, mb_detect_order(), true);
if($encoding !== "UTF-8") {
    return response()->json(json_decode(utf8_decode($data)));
}
return response()->json(json_decode($data));