I have investigated more deeply on this issue thanks to this post: Region Map does not show
The problem is that Kibana is not loading the geojson file from the webserver because of this error (found via the developer tab in my browser):
VM4472:1 Mixed Content: The page at 'https://localhost:8443/app/kibana#/visualize/create?type=region_map&indexPattern=4bd1d750-702e-11e8-a22c-d3afc294b8fc&_g=()&_a=(filters:!(),linked:!f,query:(language:lucene,query:''),uiState:(mapCenter:!(40.64730356252251,19.863281250000004),mapZoom:4,spy:!n),vis:(aggs:!((enabled:!t,id:'1',params:(),schema:metric,type:count),(enabled:!t,id:'2',params:(field:regione.keyword,missingBucket:!f,missingBucketLabel:Missing,order:desc,orderBy:'1',otherBucket:!f,otherBucketLabel:Other,size:5),schema:segment,type:te...%3Ca%20href%3D%22http:%2F%2Fwww.openstreetmap.org%2Fcopyright%22%3EOpenStreetMap%3C%2Fa%3E%20contributors%20%7C%20%3Ca%20href%3D%22https:%2F%2Fwww.elastic.co%2Felastic-maps-service%22%3EElastic%20Maps%20Service%3C%2Fa%3E%3C%2Fp%3E%26%2310;',id:road_map,maxZoom:10,minZoom:0,subdomains:!(),url:'https:%2F%2Ftiles.maps.elastic.co%2Fv2%2Fdefault%2F%7Bz%7D%2F%7Bx%7D%2F%7By%7D.png%3Felastic_tile_service_tos%3Dagree%26my_app_name%3Dkibana%26my_app_version%3D6.2.4')))),title:'New%20Visualization',type:region_map))' was loaded over HTTPS, but requested an insecure XMLHttpRequest endpoint 'http://apache:8080/regioni.geojson'. This request has been blocked; the content must be served over HTTPS.
I have configured my webserver to use HTTPS, but now I'm getting a connection timeout.
While I'm still investigating on this issue which appears to be not a Kibana misconfiguration, I'm wondering if someone knows some easier and faster method to load a geojson file.