small fish ftp can't upload image !

[myalexer]

i can't upload image into ftp
func Uper(w http.ResponseWriter , r _http.Request) {
ftp := new(ftp.FTP)
// debug default false
ftp.Debug = true
ftp.Connect(_ftp, 21)
// login
ftp.Login(*us, *ps)
if ftp.Code == 530 {
fmt.Println("error: login failure")
}

// pwd
ftp.Pwd()
fmt.Println("code:", ftp.Code, ", message:", ftp.Message)                   

// make dir
ftp.Request("TYPE I")

// stor file
bs := ioutil.ReadFile("./img_temp/myimage.png")

ftp.Stor("/images/myimage.png", bs)

ftp.Quit()      

}

result on command line :
<cmd>
<code> 220
<message> FTP Server ready.
<cmd> USER [myusername_secret]
<code> 331
<message> Password required for [myuser_secret]
<cmd> PASS [mypassword_secret]
<code> 230
<message> User [myuser_secret] logged in.
<cmd> PWD
<code> 257
<message> "/" is the current directory
code: 257 , message: "/" is the current directory
<cmd> TYPE I
<code> 200
<message> Type set to I
<cmd> PASV
<code> 227
<message> Entering Passive Mode (79,127,127,81,166,78).

i can't see error in this result !
all things ok !
upload success but image is bad !
image size before upload : 398724 byte And after upload : 0 byte !!!
why? how solved this problem?