.xltm file goes extensionless when emailing

Hello,

I have made some .xltm files with macro's and sent it to some partners.
At first when I use the Excel option to attach and send it with e-mail it worked fine. Yet, when my partners use the Excel option to attach to e-mail and send the file will lose it's extension. To go around this problem my partners save it as xlsx and then send it, yet this loses the macro's.
I need it to send it as xlsm or xltm.

Also note that it is not desireable to save the file before sending.

If anyone knows how this problem can be solved I will be delighted to hear.

Thanks,
Sander

The file manager does show extensions. Below I have 2 immages, the first of the files and the second of the email with the file attached.

It is indeed possible to add the extension once received back, but I want it to work just right away.

https://drive.google.com/open?id=0B_...kFaQWZoMGFvUUE

https://drive.google.com/open?id=0B_...zk1TTVuWS1uOW8

Could you then explain why it works on my own computer without saving.
At work I use Office 2007. My colleagees use Office 2013 just like I use on my own computer.

See video of sending without saving that works:
https://drive.google.com/open?id=0B_...XkyandjaTRBLVk