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);

        var fileProvider = new FileExtensionContentTypeProvider();
        // Figures out what the content type should be based on the file name.  
        if (!fileProvider.TryGetContentType(file.NameWithExtension, out string contentType))
        {
            throw new ArgumentOutOfRangeException($"Unable to find Content Type for file name {file.NameWithExtension}.");
        }
        
        // contentType will be application/pdf for .pdf's, application/vnd.openxmlformats-officedocument.presentationml.presentation for .pptx's, etc.
        return File(file.Contents, contentType);
    }
}

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