[BUG]Official single file upload problem

[qikong333]

Describe the bug Official single file upload problem

To Reproduce Steps to reproduce the behavior:

SaveFormFile source code

// SaveFormFile saves a result of `FormFile` to the "dest" disk full path (directory + filename).
// See `FormFile` and `UploadFormFiles` too.
func (ctx *Context) SaveFormFile(fh *multipart.FileHeader, dest string) (int64, error) {
	src, err := fh.Open()
	if err != nil {
		return 0, err
	}
	defer src.Close()

	out, err := os.Create(dest)
	if err != nil {
		return 0, err
	}
	defer out.Close()

	return io.Copy(out, src)
}

The official example

   // single file
        file, fileHeader, err:= ctx.FormFile("file")
        if err != nil {
            ctx.StopWithError(iris.StatusBadRequest, err)
            return
        }

        // Upload the file to specific destination.
        dest := filepath.Join("./uploads", fileHeader.Filename)
        ctx.SaveFormFile(file, dest)

The first argument is passed in multipart.fileHeader, but the example uses File

Expected behavior A clear and concise description of what you expected to happen.

Screenshots If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: mac

iris.Version

• go 1.16
• github.com/kataras/iris/v12 v12.2.0-alpha2.0.20210422110000-6219e5713558

Please make sure the bug is reproducible over the master branch:

$ cd PROJECT
$ go get -u github.com/kataras/iris/v12@master
$ go run .

Additional context Add any other context about the problem here.