Unable to mock mediator in controller test

[adnancindioglu]

[Authorize]
[Route("[controller]")]
[ApiController]
public class BaseApiController : Controller
{
 private IMediator _mediator;
 protected IMediator Mediator => _mediator ??= HttpContext.RequestServices.GetService<IMediator>();
}

I don't have constructor to mock mediator so i can't set my result value.
Transactions in "GetBankImageQuery" return null or false.

[ApiController]
public class BankImagesController : BaseApiController
{
        [HttpGet("getByBankId")]
        public async Task<IActionResult> GetByBankId(long bankId)
        {
            var result = await Mediator.Send(new GetBankImageQuery { BankId = bankId });
           
            if (!result.Success)
            {
                return BadRequest(result.Message);
            }
            return File(result.Data.Image, result.Data.ContentType);
      
        }
}

Unit test example in architecture:

[Test]
public async Task BankImages_GetByBankId_Success()
{
    const string requestUri = "api/BankImages/getByBankId?bankId=1";
    string token = MockJwtTokens.GenerateJwtToken(ClaimsData.GetClaims());

    var requestMessage = new HttpRequestMessage(HttpMethod.Get, requestUri);
    requestMessage.Headers.Authorization = new AuthenticationHeaderValue("Bearer", token);

    var response = await HttpClient.SendAsync(requestMessage);

    Assert.AreEqual(HttpStatusCode.OK, response.StatusCode);
}

How should I proceed here?