https://cs.chromium.org/chromium/src/chrome/browser/resources/chromeos/chromevox/cvox2/background/output.js?type=cs&q=file:output.js++listItem&sq=package:chromium&l=389
    listItem: {enter: `$name= $role $state $description`},
is relevant rule, and seems node.state was 
 {focusable:true, hovered:true}.
whereas a listBoxOption node in case of the list view mode (which is read correctly) was like
{focusable:true, hovered:true, selectable: true, selected: true}.
So I think selection status is dropped somewhere during conversion from DOM to automation node, maybe due to wrong markup.

"listItem" role is non-interactive.

file_grid.js adds "option" role to each file grid item,
https://cs.chromium.org/chromium/src/ui/file_manager/file_manager/foreground/js/ui/file_table_list.js?q=file:file_table_list.js+setAttribute&sq=package:chromium&l=141

However it's overwritten as 'listItem' by ListItem.decorate() before inserting to a list.
https://cs.chromium.org/chromium/src/ui/webui/resources/js/cr/ui/list_item.js?type=cs&q=file:list_item.js+setAttribute&sq=package:chromium&l=45

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/41cec074f9d741d8c76801779e821f07d36ca4ec

commit 41cec074f9d741d8c76801779e821f07d36ca4ec
Author: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Date: Wed Oct 11 08:58:57 2017

Markup grid view items to be recognizable as selectable option items.

Currently the role attribute is overwritten to "listItem" by
cr.ui.ListItem.decorate(). To prevent this, we need to set it when
prototype.decorate() is called, instead of the constructor.

After this change, the screenreader will be able to read an item like
"List item 2 of 5 selected" instead of "List item" when an item is
focused by it.

Bug:  772321 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: Id85596f0c8785e26184e022111b131c361e1ef47
Reviewed-on: https://chromium-review.googlesource.com/708103
Reviewed-by: Naoki Fukino <fukino@chromium.org>
Commit-Queue: Tatsuhisa Yamaguchi <yamaguchi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#507931}
[modify] https://crrev.com/41cec074f9d741d8c76801779e821f07d36ca4ec/ui/file_manager/file_manager/foreground/js/ui/file_grid.js