<form action="~/Default/PostDene" method="post" enctype="multipart/form-data">
            <textarea id="icerik" name="icerik" class="yazkutu placeholder"></textarea>
            <input type="file" class="dosya__inputfile" name="dosya">
            <button id="btngonder" class="buton">Paylaş</button>
    </form>

        public IActionResult PostDene()
        {
            return View();
        }

        [HttpPost]
        [DisableRequestSizeLimit]
        public IActionResult PostDene(IFormFile dosya)
        {
            ViewData["mesaj"] = "aaa";
            return View();
        }