Possible options for Fs.writeFile and Fs.appendFile.
Fs.writeFile
Fs.appendFile
@:optionaloptionalmode:Null<FsMode>
default = 438 (aka 0666 in Octal)
@:optionaloptionalflag:Null<FsOpenFlag>
default: 'w' for Fs.writeFile, 'a' for Fs.appendFile
@:optionaloptionalencoding:Null<String>
Encoding for writing strings. Defaults to 'utf8'. Ignored if data is a buffer