submit () {
  console.log(this.banners)
  const banners = new FormData()
  banners.append('banners', this.banners)
  this.$axios.post(api.insertBanner, banners).then(response => response.data)
    .catch(error => {
      console.log(error)
    })
},

public async Task<ActionResult> Post(BannersDTO banners)

public class BannersDTO
{
    public Microsoft.AspNetCore.Http.IFormFile Files { get; set; }
    public string TermoOrSku { get; set; }
    public bool InserirImediato { get; set; }
    public string DataAtivacao { get; set; }
    public string DataVigencia { get; set; }
}

public async Task<ActionResult> Post(Microsoft.AspNetCore.Http.IFormFile Files)

banners.append('files', this.banners.files)