FileController.cs

[Route("File")]
public class FileController : Controller
{
    private readonly IFileService _fileService;

    public FileController(IFileService fileService)
    {
        _fileService = fileService;
    }

    [HttpPost("Download/{id:guid}")]
    [ValidateAntiForgeryToken]
    public async Task<IActionResult> Download(Guid id)
    {
        // Retrieve some file from some service
        ApplicationFile file = await _fileService.GetFileAsync(id);

        string extension = Path.GetExtension(file.NameWithExtension);
        string contentType = "";

        switch (extension)
        {
            case ".pdf":
                contentType = "application/pdf";
                break;
            case ".xslx":
                contentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
                break;
            case ".docx":
                contentType = "application/vnd.openxmlformats-officedocument.wordprocessingml.document";
                break;
            default:
                throw new Exception($"Unknown extension {extension}");
        }

        return File(file.Contents, contentType);
    }
}

public class ApplicationFile
{
    public string NameWithExtension { get; set; }
    public byte[] Contents { get; set; }
}