Happened when we delete a wiki attachment during following test case:
As Wiki admin :
Go to Wiki admin page > Manage wiki attachments
Select some attachments to delete, then confirm.
Deleted attachments should not appear in the manage wiki attachments list anymore.
Deleted attachments should not appear in the list displayed to wiki writers when they add attachments.
Deleted attachments could not be downloaded when referenced in a wikipage but the link still in the page.
If the deleted attachment is a picture the picture is no more displayed in wiki pages, its name will be displayed instead.

[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "NOTICE: PHP message: PHP Fatal error: Uncaught Error: Using $this when not in object context in /usr/share/tuleap/src/common/wiki/lib/WikiAttachmentRevision.class.php:346"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "Stack trace:"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "#0 /usr/share/tuleap/src/common/wiki/lib/WikiAttachment.class.php(527): WikiAttachmentRevision::getRevisionIterator(778, NULL)"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "#1 /usr/share/tuleap/src/common/wiki/lib/WikiAttachment.class.php(332): WikiAttachment->count()"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "#2 /usr/share/tuleap/src/www/wiki/uploads.php(27): WikiAttachment->setUri('/wiki/uploads/7...')"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: "#3 {main}"
[29-Apr-2020 08:11:26] WARNING: [pool tuleap] child 25729 said into stderr: " thrown in /usr/share/tuleap/src/common/wiki/lib/WikiAttachmentRevision.class.php on line 346"