Grafana - windows - damaged UI?

First time I try to use Grafana.
I installed it (version 6.4.4) on a windows 7 machine, created a custom.ini, modify the port in 8080. The service is running.
When I access to Grafana with Firefox (version 70.0.1), with the command localhost:8080, the UI seems to be damaged:

It’s typical a HTML or css problem ?

The css and javascript files did not load. How did you install it? Did you use the Windows installer?

Can you look in the Firefox dev tools console and see what the error was? It could be a faulty installation or it could be the firewall that is blocking them from loading.

Yes, I used the msi
I checked with F12 but I didn’t see any errors: how to check that ?

Finally I found this (in French):
La feuille de style http://localhost:8080/public/build/grafana.dark.9a1523857bb65302ddff.css n’a pas été chargée car son type MIME, « text/plain », n’est pas « text/css ». localhost:8080

"The stylesheet was not loaded because its MIME type, “text/html” is not “text/css”.