Just to summarize the exchange in the comments:
org-attach-attach used to add an
Attachments property up to, and including Org mode version 9.2.4. Since version 9.2.5 (more specifically, since commit ae9cd437) this is no longer the case. Here are the details of the relevant commit (but note that there is no mention of the specific change that is the subject of this question):
Author: Gustav Wikström <firstname.lastname@example.org>
Date: Sun Nov 25 21:38:44 2018 +0100
org-attach*, org, org-manual, org-news, ox-html, testing/*
Changed the way attachments deal with property-inheritance. It now
adheres to the =org-use-property-inheritance= setting by default but
it can be customized if needed (I recommend to enable it!).
The property ATTACH_DIR is deprecated in favour of the shorter and simpler
Added an explicit option to =org-attach= for unsetting
attachment-directories (i.e. remove DIR property and deal with the
attachments by interaction).
Added attachment link type with the prefix "attachment:".
A new linktype "attachment" is added in order to reduce
link-duplication when wanting to link to files in attached folders of
nodes. This works for both ID and DIR properties. The goal is to
make the functionality for attachment links mirror the functionality
for file links.
New file, existing functionality. Code here has been factored out
from org-attach.el and if GIT-functionality is to be used this module
needs to be required sepatately. It extends org-attach by use of its
Activating git functionality in org-attach is done by loading
org-attach-git from now on, instead of customizing a variable.
Naming of both functions and tests has been modified to match the move
of functionality into its own module.
Inline images are shown also using attachment-links, exactly the same
as it works for file-links today.
Make org-open-at-point respect ARG when opening attachment-dir.
org-attach-directory has been deprecated in favour for
org-attach-id-dir. The new name matches its purpose better.
Export attachment links to images as inline images, in the same way as
file links work today.
Mention the changes in this patch.
The chapter "Refile, Copy, Archive" has been split into two separate
- "Refile, Copy and Archiving" for information related to moving
existing data around.
- "Capture, Attachments, RSS Feeds and Protocols" for information
related to working with external data.
The attachment-part has been rewritten and extended to match the
changes in this patch.
The new attachment link type is mentioned both inside the attachments
chapter and in the chapter dealing with links.
Documentation related to external links has been improved.
Require org-attach-git instead of org-attach, since this file tests
Add tests for org-attach.
Define a symbol for a file to test attachments with.
A bunch of new example files and folders are created and are used in
testing of org-attach to verify its functionality.
It is almost possible to reimplement the old functionality using the new
org-attach-after-change-hook, but not quite: the hook is run with arguments like this:
(run-hook-with-args 'org-attach-after-change-hook attach-dir)
So you could define a function that adds the property:
(defun org-add-attachment-property (file)
(let ((basename (file-name-nondirectory file)))
(when (and org-attach-file-list-property (not org-attach-inherited))
(point) "Attachments" basename))))
(add-hook 'org-attach-after-change-hook 'org-add-attachment-property)
except that the hook is run with the "wrong" argument: instead of being run with the original file, it is run with the attach directory instead, and I'm not sure why: it might be better to run it with both arguments to provide additional flexibility - run the hook like this:
(run-hook-with-args 'org-attach-after-change-hook attach-dir file)
and change the function definition so that it starts with:
(defun org-add-attachment-property (attach-dir file)
The function will ignore the
attach-dir argument and deal with the file only.
But this will require changes to Org mode code, and therefore an enhancement request to the Org mode mailing list.