How to fix a null IFormFile being accepted by an ASP.NET Web Api from a Post

Question

Ultimately I want to have a simple file upload page using as basic a jQuery code as possible with the c# code on the server but I keep receiving a null IFormFile on the server.

There are lots of examples of how this can work with an Ajax POST with Form-data and receiving an IFormFile on an ASP.NET Web API controller action. However, it seems every one of them has had to try multiple tweaks to the code to get theirs working.

I have made a simple solution with just the API part and using Postman to send a file to it. I have tried different combinations of Content-Type and Accept headers but I always receive a null file.

To satisfy some hosting requirements, I am using OWIN to self-host the application. Here is the controller code, I have used the default values controller and edited the post-action...

    public string Post([FromBody]IFormFile file)
    {
        if (file != null)
        {
            return "Success: We got something!!!";
        }
        else
        {
           return "ERROR: We got nothing :-(";
        }

     }

My StartUp.cs configuration is below. Although my example shows multipart/form-data, I have also tried application/JSON and other formats including leaving those blank.

public void Configuration(IAppBuilder app)
        {
            var config = new HttpConfiguration();
            config.Routes.MapHttpRoute("default", "{controller}");
            config.EnableCors();

            config.Formatters.JsonFormatter.SupportedMediaTypes.Add(new MediaTypeHeaderValue("multipart/form-data"));

            app.UseCors(Microsoft.Owin.Cors.CorsOptions.AllowAll);
            app.UseWebApi(config);
        }

Then here are 2 screenshots of my latest postman attempt. I have hastily put them up on a Wix server so apologies if they are not yet visible

At this point, I'll be happy for the Post method to receive a not null value for a file.

Answer 1

Use [FromForm] attribute instead of [FromBody] and remove both Accept and Content-Type headers.

EDIT:

So it appears that in haste I immediately assumed you are using ASP.NET Core (despite you pointing out otherwise) since IFormFile is Microsoft.AspNetCore.Http namespace class so naturally this explains why it doesn't work when you are trying this on .NET Framework.

I suppose technically it would be possible to create a concrete implementation of this ASP.NET Core interface and then some special case model binder, but this would certainly require some work.

I am not sure where you found these examples for making IFormFile work in .NET Framework but I could'nt find any example.

The closest experience to ASP.NET Core IFormFile binding that I could find was this ApiMultipartFormFormatter, which is super easy to implement and supports HttpFile binding. One thing that is not immediately obvious is that you will need to wrap some HttpFile property into another class to make it work.

public string Post([FromForm]MyViewModel vm)
{
    if (vm.file != null)
    {
        return "Success: We got something!!!";
    }
    else
    {
        return "Sadness: We got null :-(";
    }
}

public class MyViewModel
{
    public HttpFile SomeFile { get; set; }
}