readStream.path
流正在读取的文件的路径，如 fs.createReadStream() 的第一个参数中所指定。
如果 path 作为字符串传入，则 readStream.path 将是字符串。
如果 path 作为 <Buffer> 传入，则 readStream.path 将是 <Buffer>。
如果指定了 fd，则 readStream.path 将是 undefined。
The path to the file the stream is reading from as specified in the first
argument to fs.createReadStream(). If path is passed as a string, then
readStream.path will be a string. If path is passed as a <Buffer>, then
readStream.path will be a <Buffer>. If fd is specified, then
readStream.path will be undefined.