尝试在Golang中读取gzip文件时出错

I am trying to read gzip files using compress/gzip. I am using http.DetectContentType as I do not know if I get a normal txt file or a gzipped one. My code is very straight forward and as below:

f, err := os.Open(fullpath)
if err != nil {
    log.Panicf("Can not open file %s: %v", fullpath, err)
    return ""
}
defer f.Close()

buff := make([]byte, 512)
_, err = f.Read(buff)
if err != nil && err != io.EOF{
    log.Panicf("Cannot read buffer %v", err);
    return ""
}

switch filetype := http.DetectContentType(buff); filetype {
case "application/x-gzip":
    log.Println("File Type is", filetype)
    reader, err := gzip.NewReader(f)
    if err != nil && err != io.EOF{
        log.Panicf("Cannot read gzip archive %v", err);
        return ""
    }
    defer reader.Close()
    target := "/xx/yy/abcd.txt"
    writer, err := os.Create(target)
    if err != nil {
        log.Panicf("Cannot write unarchived file %v", err);
        return ""
    }
    defer writer.Close()

    _, err = io.Copy(writer, reader)
    return target

The problem is that the gzip reader always errors out saying "Cannot read gzip archive gzip: invalid header" I have tried the zlib library too but in vain. I gzipped the source file in mac using the command line gzip tool. Please show me where I am going wrong.