检查上传文件的类型

Question

如何FileUploader在ASP.NET C#网页中检查使用控件上载的文件的文件类型？

1. 我尝试检查文件扩展名,但是当JPEG图像(例如Leonardo.jpg)重命名为具有PDF扩展名(例如Leonardo.pdf)时,它显然会失败.

2. 我试过了

   FileUpload1.PostedFile.ContentType.ToLower().Equals("application/pdf")
   

   但由于上面的代码与第一个代码的行为方式相同,因此失败了.

有没有其他方法来检查实际的文件类型,而不仅仅是扩展名？

我看了ASP.NET如何检查文件类型的类型而不管扩展名.

编辑:我尝试从stackoverflow中的一个帖子下面的代码.但这不起作用.对此有任何想法.

/// <summary>
/// This class allows access to the internal MimeMapping-Class in System.Web
/// </summary>
class MimeMappingWrapper
{
  static MethodInfo getMimeMappingMethod;

    static MimeMappingWrapper() {
    // dirty trick - Assembly.LoadWIthPartialName has been deprecated
    Assembly ass = Assembly.LoadWithPartialName("System.Web");
    Type t = ass.GetType("System.Web.MimeMapping");

    getMimeMappingMethod t.GetMethod("GetMimeMapping", BindingFlags.Static | BindingFlags.NonPublic | BindingFlags.Public));
}

/// <summary>
/// Returns a MIME type depending on the passed files extension
/// </summary>
/// <param name="fileName">File to get a MIME type for</param>
/// <returns>MIME type according to the files extension</returns>
public static string GetMimeMapping(string fileName) {
    return (string)getMimeMappingMethod.Invoke(null, new[] { fileName });
}
}

Answer 1

不要使用File Extensions来计算MIME类型,而是使用"Winista"进行二进制分析.

假设有人exe用jpg扩展名重命名.您仍然可以确定真实的文件格式.它不检测swf或flv,但几乎所有其他众所周知的格式,你可以得到一个十六进制编辑器来添加它可以检测的更多文件.

下载Winista:在这里.

在Winista无法检测到真实文件格式的地方,我已经使用了URLMon方法:

public class urlmonMimeDetect
{
    [DllImport(@"urlmon.dll", CharSet = CharSet.Auto)]
    private extern static System.UInt32 FindMimeFromData(
        System.UInt32 pBC,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzUrl,
        [MarshalAs(UnmanagedType.LPArray)] byte[] pBuffer,
        System.UInt32 cbSize,
        [MarshalAs(UnmanagedType.LPStr)] System.String pwzMimeProposed,
        System.UInt32 dwMimeFlags,
        out System.UInt32 ppwzMimeOut,
        System.UInt32 dwReserverd
    );

public string GetMimeFromFile(string filename)
{
    if (!File.Exists(filename))
        throw new FileNotFoundException(filename + " not found");

    byte[] buffer = new byte[256];
    using (FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read))
    {
        if (fs.Length >= 256)
            fs.Read(buffer, 0, 256);
        else
            fs.Read(buffer, 0, (int)fs.Length);
    }
    try
    {
        System.UInt32 mimetype;
        FindMimeFromData(0, null, buffer, 256, null, 0, out mimetype, 0);
        System.IntPtr mimeTypePtr = new IntPtr(mimetype);
        string mime = Marshal.PtrToStringUni(mimeTypePtr);
        Marshal.FreeCoTaskMem(mimeTypePtr);
        return mime;
    }
    catch (Exception e)
    {
        return "unknown/unknown";
    }
}
}

从Winista方法内部,我回到URLMon:

   public MimeType GetMimeTypeFromFile(string filePath)
    {
        sbyte[] fileData = null;
        using (FileStream srcFile = new FileStream(filePath, FileMode.Open, FileAccess.Read))
        {
            byte[] data = new byte[srcFile.Length];
            srcFile.Read(data, 0, (Int32)srcFile.Length);
            fileData = Winista.Mime.SupportUtil.ToSByteArray(data);
        }

        MimeType oMimeType = GetMimeType(fileData);
        if (oMimeType != null) return oMimeType;

        //We haven't found the file using Magic (eg a text/plain file)
        //so instead use URLMon to try and get the files format
        Winista.MimeDetect.URLMONMimeDetect.urlmonMimeDetect urlmonMimeDetect = new Winista.MimeDetect.URLMONMimeDetect.urlmonMimeDetect();
        string urlmonMimeType = urlmonMimeDetect.GetMimeFromFile(filePath);
        if (!string.IsNullOrEmpty(urlmonMimeType))
        {
            foreach (MimeType mimeType in types)
            {
                if (mimeType.Name == urlmonMimeType)
                {
                    return mimeType;
                }
            }
        }

        return oMimeType;
    }

更新:

使用魔法来计算更多文件是一个文件签名表