It seems to work for me - this is from my view:
https://pastebin.com/raw/t2gqs0PR
(had to use PB because including it here caused the request to be blocked)
Here's from the HomeController:
[HttpPost]
public IActionResult Upload(IFormFile file) {
return Content("done");
}
Here's a breakpoint in that method:
This is a fresh sample ASP.NET Core MVC project - is this roughly what your code looks like?