Fixed

package main

import (
    "encoding/hex"
    "fmt"
    "io/ioutil"
    "log"
    "os"
    "strconv"
    "strings"
)

func main() {
    files, err := ioutil.ReadDir(".")
    if err != nil {
        log.Fatal(err)
    }

    // loop through the files
    for _, f := range files {
        hexNum := f.Name()
        index := strings.LastIndex(hexNum, "0x")
        if index == -1 {
            continue
        }
        hexNum = hexNum[index+2 : index+4]

        // Now we prefix the original name with the actual number.
        numBytes, err := hex.DecodeString(hexNum)
        if err != nil {
            fmt.Printf("Failed to contert %s to base 10: %v\n", hexNum, err)
            continue
        }

        num := bytesInt(numBytes)
        newName := strconv.FormatUint(num, 10) + " | " + f.Name()
        // now we finally rename it

        err = os.Rename(f.Name(), newName)
        if err != nil {
            fmt.Printf("%s to %s: %v\n", f.Name(), newName, err)
        }
    }
}

// return an int that is the total of bytes
func bytesInt(bytes []byte) (ret uint64) {
    for _, b := range bytes {
        ret += uint64(b)
    }

    return ret
}