How do I add data to an interface that is taken as an argument in Go?

I'm just picking up go, so apologies if my terminology isn't precise. My end goal is to add the name of a cachebusted CSS file to the layout template of my Go application. The CSS file built on the fly when the application starts up so can't be hardcoded. In my template file I have this:

//More html here
<link href="{{.CSSFile}}" rel="stylesheet">
//more html here 

I have a Render method on a View type like shown below. It takes data interface{} as an argument and then runs ExecuteTemplate. It is called by every controller in one way or another that sends the data argument and exposes information. I know how to add it as data from the controller that then calls the Render method, but I obviously don't want to add the CSS file in every single controller action, so it makes the most sense to add it in the Render function one time and have it added to the data that gets passed to ExecuteTemplate. My issue is how do I add this information to the data already being passed to Render and then pass that whole of information to ExecuteTemplate. What I have below works for the CSS file, but it obviously doesn't send along the data that was passed to the original Render argument.

type View struct {
	Template *template.Template
	Layout   string
}


func (v *View) Render(w http.ResponseWriter, data interface{}) error {
	d := Data{}
	d.AddCSSFile()
	w.Header().Set("Content-Type", "text/html")
	err := v.Template.ExecuteTemplate(w, v.Layout, d)
	if err != nil {
		log.Println(err)
		fmt.Fprintln(w, "<h1>Something went wrong. Please contact us at support")
	}
	return nil
}

type Data struct {
	Alerts  []Alert
	Yield   interface{}
	CSSFile interface{}
}


func (d *Data) AddCSSFile() {
	ss, _ := filepath.Glob("./assets/site-*.css")
	fp := strings.Join(ss, "")
	_, d.CSSFile = filepath.Split(fp)
}

I've created a gist which, not entirely complete, is a little more fleshed out of what I'm trying to do:
https://gist.github.com/codelitt/549a68149add0482c6dc2514a46aa580

Not sure I understand exactly what you're asking but if what you want is to combine the data interface{} argument with the d := Data{} value inside Render, then you could do something like this...

// ...
func (v *View) Render(w http.ResponseWriter, data interface{}) error {
p := Page{Data:data}
p.AddCSSFile()
w.Header().Set("Content-Type", "text/html")
err := v.Template.ExecuteTemplate(w, v.Layout, p)
if err != nil {
log.Println(err)
fmt.Fprintln(w, "<h1>Something went wrong. Please contact us at support")
}
return nil
}
type Page struct {
Alerts  []Alert
Yield   interface{}
CSSFile interface{}
Data interface{}
}
func (p *Page) AddCSSFile() {
// ...
}

Edit: Or you could also just initialize an anonymous struct value and pass it to ExecuteTemplate without having to change you existing Data type.

// ...
err := v.Template.ExecuteTemplate(w, v.Layout, struct{
Data
Args interface{}
}{Data:d, Args:data})
// ...

Edit2: So if I understand your comment correctly the data interface{} argument passed to the Render method could in some or all instances be or contain a value of a type that matches one of the Data fields' types, in which case you would like to set that value to that field so as to pass it together to the ExecuteTemplate method. At least one solution to that, as you've already found out, is to use type assertion. Below is a slightly modified version of your example from the comment in the context of your original example from the question.

func (v *View) Render(w http.ResponseWriter, data interface{}) error {
d := Data{}
d.AddCSSFile()
if alerts, ok := data.([]Alert); ok {
d.Alerts = alerts
}
w.Header().Set("Content-Type", "text/html")
err := v.Template.ExecuteTemplate(w, v.Layout, d)
if err != nil {
log.Println(err)
fmt.Fprintln(w, "<h1>Something went wrong. Please contact us at support")
}
return nil
}