Hi Mike,
The problem appears to be that the dash after the C or F is getting turned into an underscore ("_")
I've just tried sending a message to one of my test subgroups with an attachment called C-Test1.lpf and it gets sent back to me and my other test account as that.? There is no renaming going on that I can see.
In an effort to eliminate the obvious, can you confirm that you are looking at an attachment when you receive a message and not just a link to it?? The settings to determine whether groups.io sends a file or a link are in the Advanced Preferences of the member settings for each group.
Regards,
Andy
Andy Wedge