fix bug: upload more than 2 files in a single post request will be failed

In the function getTotalLength, the length of a single file lost 2 bytes because of the CR_LF.
When uploading 1 file in a single post request, lost 2 bytes, the CR_LF will not be sent.
When uploading 2 file in a single post request, lost 4 bytes, the substring in file split "--" will not be sent.
When uploading more than 2 file in a single post request, lost more than 4 bytes, the last file split will not be complete any more. The server will show an error.