UPDATE: It is reported that Google have issued a new Chrome update which fixes the issue, we are currently running Version 83.0.4103.97.
RESOLVED: This issue has now been addressed by our software engineers. To resolve on your installation, download the attached tgz and apply to the root directory of your atmail installation. If you have any issues with this, please submit a ticket to speak with atmail support staff.
Google has recently updated Chrome to Version 83.0.4103.61 and part of this update has stopped the ability to add attachments using the Attach from computer button when you send an email, Chrome errors with "Error occurred uploading attachment: "
There are two workarounds for this issue
Dragging and dropping attachments on to the email still works in Chrome
use a different browser, others browsers are working as expected.
atmail 8.6 and 8.7 are not affected by the Chrome update.