How to Model Object upload in API with form-data in asp.net Web Api

You Can handle the situation with two scenario.

1. You can use HttpContext.Current.Request

• Not required as you mentioned in model class byte[] property.

Ref. Link 1 : https://www.c-sharpcorner.com/UploadFile/2b481f/uploading-a-file-in-Asp-Net-web-api/

public class ValuesController: ApiController
{
[System.Web.Http.HttpPost]
    public HttpResponseMessage PostComplex(TestAspDotNetModelClass update)
    {
        if (update.fileProfile != null)
        {
           HttpResponseMessage result = null;  
           var httpRequest = HttpContext.Current.Request;  
           if (httpRequest.Files.Count > 0)  
            {  
                var docfiles = new List<string>();  
                foreach (string file in httpRequest.Files)  
                {  
                    var postedFile = httpRequest.Files[file];  
                    var filePath = HttpContext.Current.Server.MapPath("~/" + postedFile.FileName);  
                    postedFile.SaveAs(filePath);  
                    docfiles.Add(filePath);  
                }  
                result = Request.CreateResponse(HttpStatusCode.Created, docfiles);  
            }  
            else  
            {  
                result = Request.CreateResponse(HttpStatusCode.BadRequest);  
            }  
            
        } 
        else
        {
            return Request.CreateResponse(HttpStatusCode.BadRequest);
        }
    }
 }

2. You can go with same as you mentioned byte[] that is not possible to create json thats why you can convert byte[] to Base64String then it is possible.

Ref. Link 2 : Saving a base64 string as an image into a folder on server using C# and Web Api

Model Class

public class TestAspDotNetModelClass
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string Base64Stringfile { get; set; }
}

API Controller Action

public class ValuesController: ApiController
{
[System.Web.Http.HttpPost]
    public HttpResponseMessage PostComplex(TestAspDotNetModelClass update)
    {
        if (update.Base64Stringfile != null)
        {
            // Convert base 64 string to byte[]
            byte[] fileBytes = Convert.FromBase64String(update.Base64Stringfile);
            // Convert your byte[] to file
            return Request.CreateResponse(HttpStatusCode.OK);
        } 
        else
        {
            return Request.CreateResponse(HttpStatusCode.BadRequest);
        }
    }
}