Unit Test WebApi to upload image using Mock

I am starting to write unit tests for all our controllers and seem to be getting the hang of it, but now i am a bit stuck. I have the following controller method that i would like to write a unit test for but a am a bit lost. Can someone please help and point me in the right direction. I am guessing maybe i need to abstract the method slightly, but not sure how.

public async Task<IHttpActionResult> PostAsync()
{
    if (HttpContext.Current.Request.Files.AllKeys.Any())
    {
        // Get the uploaded image from the Files collection
        var httpPostedFile = HttpContext.Current.Request.Files[0];

        if (httpPostedFile != null)
        {
            // Validate the uploaded image, by only accepting certain file types and sizes
            var validExtensions = new List<string>
            {
                ".JPG", ".JPE", ".BMP", ".GIF", ".PNG"
            };

            if (!validExtensions.Contains(Path.GetExtension(httpPostedFile.FileName).ToUpperInvariant()))
            {
                return BadRequest();
            }
            else if (httpPostedFile.ContentLength > 2097152)
            {
                // file is over 2mb in size
                return BadRequest();
            }

            // create a new image
            var entity = new Image
            {
                Name = httpPostedFile.FileName,
                Size = httpPostedFile.ContentLength,
                Data = new ImageData
                {
                    Content = new byte[httpPostedFile.ContentLength]   
                }
            };

            await httpPostedFile.InputStream.ReadAsync(entity.Data.Content, 0, httpPostedFile.ContentLength);

            await _service.AddAsync(entity);

            return Created<ImageModel>(Request.RequestUri, Mapper.Map<ImageModel>(entity));
        }
    }

    return BadRequest();

}