Question

Is there a way to use the HttpPostedFileBase elements to find out if its a valid video file fomart (except of validating the extension). Im using aspnet mvc 3 by the way.

Answer 1

If you want to be absolutely certain, it is best to check the stream of the HttpPostedFileBase for magic bytes. This is because some applications may write it as an extension that you think you can process (such as MP4), but in reality it is another format such as M4V.

For example, to check if a stream is an MP4 variation stream, you can check if the stream starts with the bytes 0x00, 0x00, 0x00, 0x20, 0x66, 0x74, 0x79, 0x70, 0x6D, 0x70 and 0x34. You can find more formats here.

Something like this may work:

public static bool IsMP4(System.IO.Stream stream)
{
    return HasMagicBytes(stream, 0, 0x00, 0x00, 0x00, 0x20, 0x66, 0x74, 0x79, 0x70, 0x6D, 0x70, 0x34);
}

public static bool HasMagicBytes (System.IO.Stream stream, int offset, params byte[] magicBytes)
{
    try {
        bool match = false;
        byte[] bytes = new byte[magicBytes.Length];
        if (stream.Read (bytes, offset, magicBytes.Length) == magicBytes.Length) {
            for (int i = 0; i < magicBytes.Length; i++) {
                if (bytes [i] != magicBytes [i]) {
                    return false;
                }
            }   
            return true;
        } else {
            return false;
        }

    } finally {
        stream.Seek (0, System.IO.SeekOrigin.Begin);
    }

}

Some care should be taken whether the stream is seekable. This technique is rather efficient for larger files. To make this reusable, you can write a ValidationAttribute so it may be possible to define your model like this:

public class MyModel
{
    [CheckFormat(0x00, 0x00, 0x00, 0x20, 0x66, 0x74, 0x79, 0x70, 0x6D, 0x70, 0x34)]
    public HttpPostedFileBase MyFile { get; set; }
}

But I’ll leave that to you to investigate further.