说明：  大文件拷贝之所以用文件流来进行拷贝，主要是由于如果用File静态类来执行拷贝就是将整个文件整体传输，对于一个好几个G的大文件会造成内存占用大，运行慢，效率不高。所以用到文件流拷贝。文件流拷贝可以设置拷贝的二进制流缓冲区的大小，然后根据缓冲区的大小来一点一点拷贝，就类似与U盘拷贝文件到电脑似的。
(Large File copy File stream is used to copy, mainly because if you use the File static class to perform the copy is to transmit the entire File as a whole, for a large File can cause several G memory footprint, run slow, efficiency is not high.So file stream copy is used.A file stream copy can set the size of the binary stream buffer of the copy, and then copy it bit by bit according to the size of the buffer, similar to a usb drive copying a file to a computer.)