When attaching a file to an Email quick message using scripts (for example, the GlideSysAttachment.copy method), you cannot remove the attachment.
Steps to Reproduce
Add an attachment to an incident.
Create a business rule on sys_email table that contains the following code that runs before insert:
GlideSysAttachment.copy('incident', 'd71f7935c0a8016700802b64c67c11c6', 'sys_email', current.sys_id);
Open a non-closed incident and open the Email client.
Note that the Email Client shows a file attached because of the script.
Click Manage Attachments or the Attachments (paperclip) icon and remove the attached file.
Note that the attached file is not removed from the Quick Message form and it will be emailed.
Instead of using a script to add attachments to sent emails, use the Manage Attachments options on the Email client.
Related Problem: PRB1245823