Golang如何实现表单文件上传限制

func uploadHandler(w http.ResponseWriter, r *http.Request) {
    // 允许内存中最多10MB，整个请求不超过20MB
    err := r.ParseMultipartForm(10 << 20)
    if err != nil {
        if err == http.ErrContentLengthExceeded {
            http.Error(w, "上传文件过大", http.StatusBadRequest)
            return
        }
        http.Error(w, "解析表单失败", http.StatusInternalServerError)
        return
    }

    file, handler, err := r.FormFile("uploadFile")
    if err != nil {
        http.Error(w, "获取文件失败", http.StatusBadRequest)
        return
    }
    defer file.Close()

    // 检查文件实际大小
    if handler.Size > 20<<20 {
        http.Error(w, "文件不能超过20MB", http.StatusBadRequest)
        return
    }

    // 正常处理文件...
}

fileBytes := make([]byte, 512)
_, err = file.Read(fileBytes)
if err != nil {
    http.Error(w, "读取文件出错", http.StatusInternalServerError)
    return
}

contentType := http.DetectContentType(fileBytes)
allowedTypes := map[string]bool{
    "image/jpeg": true,
    "image/png":  true,
    "image/gif":  true,
}
if !allowedTypes[contentType] {
    http.Error(w, "不支持的文件类型", http.StatusBadRequest)
    return
}

// 注意：Read后需要重置文件指针
file.Seek(0, 0)

files := r.MultipartForm.File["uploadFiles"]
if len(files) > 5 {
    http.Error(w, "最多上传5个文件", http.StatusBadRequest)
    return
}