Log request in ASP.NET Web Api

I try to log all posted data in my WebApi when a exception is thrown. I have try this:

protected async override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
{
    if (request.Content == null) 
        return await base.SendAsync(request, cancellationToken);

    var Body = await request.Content.ReadAsStringAsync();

    request.Properties["body"] = Body;

    return await base.SendAsync(request, cancellationToken);
}

And in my error log method i get the posted values in Properties["body"].

But when the users upload a image I get this when I try to get the files:

This method or property is not supported after HttpRequest.GetBufferlessInputStream has been invoked.
at System.Web.HttpRequest.get_Files()

How can I store only posted data(text) in this property?