几种选择：

您可以检查文件扩展名：

static bool HasJpegExtension(string filename)
{
    // add other possible extensions here
    return Path.GetExtension(filename).Equals(".jpg", StringComparison.InvariantCultureIgnoreCase)
        || Path.GetExtension(filename).Equals(".jpeg", StringComparison.InvariantCultureIgnoreCase);
}

或检查文件标题中正确的幻数：

static bool HasJpegHeader(string filename)
{
    using (BinaryReader br = new BinaryReader(File.Open(filename, FileMode.Open, FileAccess.Read)))
    {
        UInt16 soi = br.ReadUInt16();  // Start of Image (SOI) marker (FFD8)
        UInt16 marker = br.ReadUInt16(); // JFIF marker (FFE0) or EXIF marker(FFE1)

        return soi == 0xd8ff && (marker & 0xe0ff) == 0xe0ff;
    }
}

另一种选择是加载图像并检查正确的类型。但是，这效率较低（除非无论如何都要加载图像），但可能会为您提供最可靠的结果（请注意加载和解压缩以及可能的异常处理的额外费用）：

static bool IsJpegImage(string filename)
{
    try
    {
        using (System.Drawing.Image img = System.Drawing.Image.FromFile(filename)) 
        {           
            // Two image formats can be compared using the Equals method
            // See http://msdn.microsoft.com/en-us/library/system.drawing.imaging.imageformat.aspx
            //
            return img.RawFormat.Equals(System.Drawing.Imaging.ImageFormat.Jpeg);
        }
    }
    catch (OutOfMemoryException)
    {
        // Image.FromFile throws an OutOfMemoryException 
        // if the file does not have a valid image format or
        // GDI+ does not support the pixel format of the file.
        //
        return false;
    }
}