We are currently experiencing a bug in altmail whereby if there is a significant delay between adding an attachment and sending (say 3+ minutes) altmail can sometimes fail to send the attachment. We are currently investigating the issue.
In the mean time in order to avoid this issue we recommend using webmail.blacknight.com instead. On the login screen, select the option for 'simple interface' in order to avoid any compatibility issues.
Alternatively you can set the account up in a mail client like Outlook which won't have this issue, as per: