Check file extension before uploading file in asp.net web api

Question

Here is my task:

1. I need to upload immages to a server using Asp.Net web api.

2. I need to check file's extension before uploading it (I want to allow uploading only immages).

3. I need to get the file as a Stream or Base64String as I want to resize it before saving to server.

Here is what I've tried. I am able to upload file to server, using MultipartFormDataStreamProvider, and after I've inhereted my CustomMultipartFormDataStreamProvider from that MultipartFormDataStreamProvider, I was able to check file-extension in the GetStream method like:

public override Stream GetStream(HttpContent parent, System.Net.Http.Headers.HttpContentHeaders headers)
{             
    if (!String.IsNullOrEmpty(headers.ContentDisposition.FileName))
    {
        var fileExtension = CommonUtils.GetFileExtension(headers.ContentDisposition.FileName);
        if (_allowedExtensions != null && !_allowedExtensions.Contains(fileExtension.ToLower()))
        {
            return Stream.Null;
        }    
    }

    return base.GetStream(parent, headers);
}

But MultipartFormDataStreamProvider saves file to the specified folder and not returning a Stream, and if I want to resize it, I need to read it from the HDD, resize, save as new file and delete the old one.

The other variant is to use MultipartMemoryStreamProvider - here I can get a Stream:

var task = Request.Content.ReadAsMultipartAsync<MultipartMemoryStreamProvider>(new MultipartMemoryStreamProvider())
            .ContinueWith<HttpResponseMessage>((tsk) =>
        {
            MultipartMemoryStreamProvider provider = tsk.Result;

            Stream stream = provider.Contents[0].ReadAsStreamAsync().Result;
            String imageBase64 = Convert.ToBase64String(CommonUtils.StreamToByteArray(stream));
            ......
        }

But with MultipartMemoryStreamProvider I don't know how to check file extension as I can't override method GetStream

Is there a way to both check file extension before uploading file and get a file as a Stream instead of saving it to the disc. Or the only way is to save->resize->save new->delete old?

Answer 1

I don't know how to check file extension as I can't override method GetStream

And why is that? Nothing stops you from overriding GetStream

For example:

public class ImageOnlyMultipartMemoryStreamProvider : MultipartMemoryStreamProvider
{
    public override Stream GetStream(HttpContent parent, HttpContentHeaders headers)
    {

         var fileExtension = CommonUtils.GetFileExtension(headers.ContentDisposition.FileName);

         return _allowedExtensions == null || _allowedExtensions.Any(i => i.Equals(fileExtension , StringComparison.InvariantCultureIgnoreCase)) ? base.GetStream(parent, headers) : Stream.Null;
    }
}