Add an icon to "Install new extensions" in the providers popup menu.

ui/file_manager/file_manager/foreground/css/file_types.css

Index: ui/file_manager/file_manager/foreground/css/file_types.css
diff --git a/ui/file_manager/file_manager/foreground/css/file_types.css b/ui/file_manager/file_manager/foreground/css/file_types.css
index f697876719569f297a6ba16f0d11e7622be7df42..1ddfca7335b40e5fbe337b3c93bac047a3afdc6d 100644
--- a/ui/file_manager/file_manager/foreground/css/file_types.css
+++ b/ui/file_manager/file_manager/foreground/css/file_types.css
@@ -325,16 +325,22 @@ tree .tree-item[selected] > .tree-row > .shared[file-type-icon='folder'] {
 }
 
 .tree-row:not([selected]) [volume-type-icon='provided'],
-cr-menu-item .menu-icon {
+cr-menu-item.providers-menu-item .menu-icon {
   /* Apply the rgb(90, 90, 90) mask. */
   -webkit-filter: contrast(0) brightness(0.7143);
 }
 
-cr-menu-item[selected] .menu-icon {
+cr-menu-item.providers-menu-item[selected] .menu-icon {
   /* Apply the rgb(255, 255, 255) mask. */
   -webkit-filter: contrast(0) brightness(2);
 }
 
+cr-menu-item.providers-menu-item[command='#install-new-extension'] .menu-icon {
+  background-image: -webkit-image-set(
+      url(../images/files/ui/store.png) 1x,
+      url(../images/files/ui/2x/store.png) 2x);
+}
+
 .tree-row [menu-button-icon='add-new-services'] {
   background-image: -webkit-image-set(
       url(../images/volumes/add.png) 1x,