This issue is caused by a bug in the iOS mail client rather than MindGenius and stems from the length of your filename.
The iOS mail client will convert any file with a name longer than 60 characters to a .dat extension.
There are two workarounds available. Use workaround one if you're able to rename your map on the iPad. Use workaround two if your map name needs to be longer than 60 characters.
Workaround One involves renaming the map on the iPad and then resending:
- Tap the "Maps" button
- Tap "Edit"
- Tap the map whose name is longer than 60 characters
- Edit the map name to be < 60 characters
- Tap "Save"
- Resend your map
Workaround Two involves changing the file extension after it's been sent to the desktop:
- Save the .dat file to your desktop
- Open Windows Explorer by typing the "Windows"+"E" key combination
- On the toolbar just under the address bar, open the "Organize" menu and select "Folder and search options"
- Switch to the "View" tab
- Scroll down the list until you find "Hide extensions for known file types" and uncheck it
- Click "OK"
- Navigate to your map file
- Edit the name of the file by changing its extension from .dat to .mgmx
- Accept the warning dialog, then open the file as normal