[ApiController]
    [Route("[controller]")]
    public class FileController : ControllerBase
    {
        private readonly IWebHostEnvironment _hosting;

        public FileController( IWebHostEnvironment hosting)
        {       
            this._hosting = hosting;
        }   

        [HttpGet("Download")]
        public async Task<IActionResult> Download()
        {         
                if (string.IsNullOrWhiteSpace(_hosting.WebRootPath))
                {
                    _hosting.WebRootPath = Path.Combine(_hosting.ContentRootPath, "wwwroot");
                }
                string myFilePath = _hosting.WebRootPath + @"\test.jpg";
                string fileName = "test.jpg";
                byte[] fileBytes = await System.IO.File.ReadAllBytesAsync(myFilePath);
                return File(fileBytes, "application/octet-stream", fileName);    
       
        }
    }

window.SaveFile = (url, fileName) => {
    const anchorElement = document.createElement('a');
    anchorElement.href = url;
    anchorElement.download = fileName ?? '';
    anchorElement.click();
    anchorElement.remove();
}

@inject HttpClient Http
@inject IJSRuntime _JS
@page "/"

<button class="btn btn-info" @onclick="Download">File</button>

@code{
    protected async Task Download()
    {
        var apiAddress = Http.BaseAddress + "File/Download";

        await _JS.InvokeVoidAsync("SaveFile", apiAddress, "test.jpg");
    }
}