iOS10 native mail app and attachments

iOS and iPadOS

Hello all,
Is anyone here having issues with attachments in the native iOS 10 mail app?
Specifically I'm having trouble importing and exporting files from the Myway Classic GPS app. For those unfamiliar with this app it is possible to create custom routes containing a list of coordinates that can be exported/imported to another device via an email attachment. The attached file has an .plist extension.
When I open the mail containing the plist file I am unable to locate the attachment despite the fact that voiceover announces that the mail contains an attachment.
Using my retired iPhone running iOS9.3.5 I can open the same mail, tap on the attachment and get a pop-up with one of the choices being "copy to Myway Classic".
Interestingly, other attachment types such as .pdf files behave as expected.