fix(internal): register mime type for .mjs files (#577)

Closes #575

I'm gonna be totally honest, I'm not sure if this is needed. However,
measure twice, cut once.

Signed-off-by: Xe Iaso <me@xeiaso.net>

internal/mimetype.go

@@ -0,0 +1,7 @@
+package internal
+
+import "mime"
+
+func init() {
+	mime.AddExtensionType(".mjs", "text/javascript")
+}