Function portable_name 

fn portable_name(name: &str) -> String

A file name from elsewhere (an attachment’s), as every system takes it: the folder travels to Windows and macOS too. Its last part only, back slashes included; what Windows refuses (“:” would even write into a hidden stream) as “-”; no final dot or space; a device name (“CON”, “NUL”…) with a “_” before it; “paper” when nothing is left.