Routing controller to static files

I need a controller that will return static HTML, CSS, js, and other files. I tried to figure out how routing works, but I failed.

I've tried setting up routing like this:

app.MapControllerRoute(name: "page", pattern: "Page/{pageName?}",
    defaults: new { controller = "Page", action = "Index" });

I have a PageController with a single method Index(string fileName) and I want that when requesting any HTML page, any js, CSS, and other files, the request would process the Index method.

Example:

https://localhost:7177/ - will return index.html,

https://localhost:7177/indexStyles.css - will return indexStyles.css,

https://localhost:7177/indexScript.js - will return indexScript.js,

https://localhost:7177/user.html - will return user.html and so on.

That said, I don't want to use static files app.UseStaticFiles().
The request to such pages looks terrible because of the .html on the end.

thanks to @Qing Guo. My final code looks like this:

Program.cs:

app.MapControllerRoute(name: "page", pattern: "{fileName?}",
defaults: new { controller = "Page", action = "Index" });

and PageController class:

public IActionResult Index(string fileName)
{
    if (fileName == null)
    {
        return File("index.html", "text/html");
    }
    string fileFormat = Path.GetExtension(fileName);

    if (fileFormat == "")
       fileName += ".html";

    fileFormat = Path.GetExtension(fileName);

    if (System.IO.File.Exists($"wwwroot/{fileName}"))
    {
        string contentType = $"text/{fileFormat[1..]}";

        if (fileFormat == ".jpg")
            contentType = "image/jpg";
           
        return File($"{fileName}", contentType);
    }
    else
    {
        return NotFound();
    }
}