Mobile Print will not open the attachment which is a "message" and process the files within it. There could be multiple "message" attachments in an email with many files within it. Mobile Print would not know which ones were intended to be processed and which ones were not. Additionally opening the message itself could be a dangerous.
The problem here is how the mail client is forwarding the attachment. To add to the problem it may vary based on the version of the Android OS in use and the mail client/server system being used. After briefly reading through a few articles on the internet not all Android version have the same settings that can be used based on the mail system being used.
The user should be able to "Save" the attachment first then add it to an email to be sent to Mobile Print.