Filter file type to upload in ASP.Net Generic Handlers

Question

I would like to upload some files with jQuery File Upload, but i want to filter file types by this code:

public class file : IHttpHandler
{

    public void ProcessRequest(HttpContext context)
    {
            HttpPostedFile postedFile = context.Request.Files["file"];
            string extension = Path.GetExtension(postedFile.FileName).ToLower();
            string[] validExtensions = { ".jpg", ".jpeg", ".png", ".gif", ".pdf", ".doc", ".docx", ".xls", ".xlsx", ".ppt", ".pptx", ".pps", ".ppsx" };
            if (extension.IndexOf(extension) != -1) 
            {
                        // upload files here
            }
    }

    public bool IsReusable
    {
        get
        {
            return false;
        }
    }

}

this code works well, but when i rename file extensions (for example when rename x.exe to x.jpg) the above code, accept the file type and start to upload file.

How can i handle this problem?

Answer 1

You can't be absolutely sure that a file is of type X or Y (in your case an image) unless you try to open the posted file.

Nevertheless, you can do some test against your HttpPostedFile

I use the following code to do so

bool IsImage(string fileName, string contentType, Stream stream)
{
    if (!LooksLikeAnImage(fileName, contentType)) 
        return false;
    try { 
        using (Image image = Image.FromStream(stream)) 
            return true; 
    }
    catch { 
        return false; 
    }
}

bool LooksLikeAnImage(string fileName, string contentType)
{
    if (fileName != null)
    {
        if (Perceived.GetPerceivedType(fileName).PerceivedType == PerceivedType.Image) 
            return true;
        string ct = ContentType.GetRegistryContentType(fileName);
        if (ct != null && ct.StartsWith("image/")) 
            return true;
    }
    return contentType != null && contentType.StartsWith("image/");
}

Hope this helps.