如何指定Content-SHA256?

package main
import (
    "crypto/sha256"
    "fmt"
    "io"
    "os"
    obs "github.com/huaweicloud/huaweicloud-sdk-go-obs/obs"
)
func sha256File(path string) string {
    file, err := os.Open(path) // Open the file for reading
    if err != nil {
        panic(err)
    }
    defer file.Close() // Be sure to close your file!
    hash := sha256.New() // Use the Hash in crypto/sha256
    if _, err := io.Copy(hash, file); err != nil {
        panic(err)
    }
    return fmt.Sprintf("%x", hash.Sum(nil))
}
func main() {
    //推荐通过环境变量获取AKSK，这里也可以使用其他外部引入方式传入，如果使用硬编码可能会存在泄露风险。
    //您可以登录访问管理控制台获取访问密钥AK/SK，获取方式请参见https://support.huaweicloud.com/usermanual-ca/ca_01_0003.html。
    ak := os.Getenv("AccessKeyID")
    sk := os.Getenv("SecretAccessKey")
    // 【可选】如果使用临时AK/SK和SecurityToken访问OBS，同样建议您尽量避免使用硬编码，以降低信息泄露风险。您可以通过环境变量获取访问密钥AK/SK，也可以使用其他外部引入方式传入。
    securityToken := os.Getenv("SecurityToken")
   // endpoint填写Bucket对应的Endpoint, 这里以华北-北京四为例，其他地区请按实际情况填写。
    endPoint := "https://obs.cn-north-4.myhuaweicloud.com"
    // 创建obsClient实例
    // 如果使用临时AKSK和SecurityToken访问OBS，需要在创建实例时通过obs.WithSecurityToken方法指定securityToken值。
    obsClient, err := obs.New(ak, sk, endPoint, obs.WithSecurityToken(securityToken), obs.WithSignature(obs.SignatureObs))
    if err != nil {
        fmt.Printf("Create obsClient error, errMsg: %s", err.Error())
    }
    input := &obs.PutObjectInput{}
    // 指定存储桶名称
    input.Bucket = "examplebucket"
    // 指定下载对象，此处以 example/objectname 为例。
    input.Key = "example/objectname"
    filePath := "localfile"
    fd, _ := os.Open(filePath)
    defer fd.Close()
    input.Body = fd
    // 流式上传本地文件
    output, err := obsClient.PutObject(input, obs.WithCustomHeader("x-obs-content-sha256", sha256File(filePath)))
    if err == nil {
        fmt.Printf("Put object(%s) under the bucket(%s) successful!\n", input.Key, input.Bucket)
        fmt.Printf("StorageClass:%s, ETag:%s\n",
            output.StorageClass, output.ETag)
        return
    }
    fmt.Printf("Put object(%s) under the bucket(%s) fail!\n", input.Key, input.Bucket)
    if obsError, ok := err.(obs.ObsError); ok {
        fmt.Println("An ObsError was found, which means your request sent to OBS was rejected with an error response.")
        fmt.Println(obsError.Error())
    } else {
        fmt.Println("An Exception was found, which means the client encountered an internal problem when attempting to communicate with OBS, for example, the client was unable to access the network.")
        fmt.Println(err)
    }
}