public async Task<HttpResponseMessage> PostFile()
{
    // Check if the request contains multipart/form-data.
    if (!Request.Content.IsMimeMultipartContent())
    {
        throw new HttpResponseException(HttpStatusCode.UnsupportedMediaType);
    }

    //string root = HttpContext.Current.Server.MapPath("~/App_Data");
    string root = HttpContext.Current.Server.MapPath("~/Files");
    var provider = new MultipartFormDataStreamProvider(root);

    try
    {
        StringBuilder sb = new StringBuilder(); // Holds the response body

        // Read the form data and return an async task.
        await Request.Content.ReadAsMultipartAsync(provider);

        // This illustrates how to get the form data.
        foreach (var key in provider.FormData.AllKeys)
        {
           // Trace.WriteLine(key.Headers.ContentDisposition.FileName);
            foreach (var val in provider.FormData.GetValues(key))
            {
                sb.Append(string.Format("{0}: {1}n", key, val));
            }
        }

        // This illustrates how to get the file names for uploaded files.
        foreach (var file in provider.FileData)
        {
            FileInfo fileInfo = new FileInfo(file.LocalFileName);
            sb.Append(string.Format("Uploaded file: {0} ({1} bytes)n", fileInfo.Name, fileInfo.Length));
        }
        return new HttpResponseMessage()
        {
            Content = new StringContent(sb.ToString())
        };
    }
    catch(System.Exception e)
    {
        return Request.CreateErrorResponse(HttpStatusCode.InternalServerError, e);
    }
}

using System.Net.Http;
using System.Net.Http.Headers;

...

public class CustomMultipartFormDataStreamProvider
    : MultipartFormDataStreamProvider
{
    public CustomMultipartFormDataStreamProvider(string path)
        : base(path)
    {
    }

    public override string GetLocalFileName(HttpContentHeaders headers)
    {
        return headers.ContentDisposition.FileName.Replace(""", string.Empty);
    }
}

var provider = new MultipartFormDataStreamProvider(root);

var provider = new CustomMultipartFormDataStreamProvider (root);