Can't attach files to emails anymore from My Cloud

The My Cloud Desktop program has been end of life since 2016. WD Support even has a Knowledgebase article covering the program being discontinued.

https://support-en.wd.com/app/answers/detail/a_id/25159

One possible workaround is to downgrade the firmware and see if that fixes the issue since it was the latest firmware that broke the Desktop program. Otherwise your stuck with using the MyCloud.com web portal.

There is some information on how to downgrade the firmware along with past firmware links in the following post.