FileController.cs

[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);
}