'HttpPostedFileBase' in Asp.Net Core 2.0

HttpPostedFileBase doesn't exist in ASP.NET Core. You should use IFormFile now, instead. However, that only works when you send the request as multipart/form-data, which you're likely not doing if you're working with a client-side framework like React. If you're posting JSON, you should set the JSON member that corresponds with your file property with the file encoded as a Base64 string. Server-side, you should then bind to byte[].

You don't have HttpPostedFileBase in ASP.NET Core 2.0, but you can use IFormFile.

[HttpPost("UploadFiles")]
public async Task<IActionResult> Post(List<IFormFile> files)
{
    long size = files.Sum(f => f.Length);

    // full path to file in temp location
    var filePath = Path.GetTempFileName();

    foreach (var formFile in files)
    {
        if (formFile.Length > 0)
        {
            using (var stream = new FileStream(filePath, FileMode.Create))
            {
                await formFile.CopyToAsync(stream);
            }
        }
    }

    // process uploaded files
    // Don't rely on or trust the FileName property without validation.

    return Ok(new { count = files.Count, size, filePath});
}

More here: https://docs.microsoft.com/en-us/aspnet/core/mvc/models/file-uploads?view=aspnetcore-2.1

IFormFile is in the following namespace Microsoft.AspNetCore.Http.