When using a Secure Email Gateway (SEG) with attachment encryption enabled, attachments that have a file name longer than 70 characters will show as a mime-attachment in the email message. These attachments cannot be properly opened.
If the file name is shortened to less than 70 characters, the file can be attached and opened properly.
This issue has been given identifier CMEM-181548. Our product team has been engaged and is working on a resolution.
If your SEG is at least on version 8.2, adding a space in the file name of the attachment will allow it to be properly included in the email message.