Hi Anish,
The information I have shared earlier provide the exact reasons that can be explained to your customers. You can explain the role of PR_ATTACH_FILENAME and PR_ATTACH_EXTENSION in determining the availability of file extension. As I have mentioned earlier as well, the absence of these properties represent the absence of corresponding MapiMessage’s property and these can be used for checking the condition in code. Please let us know if we can be of any additional help to you in this regard.