Can't attach particular files

I don't think anything changed in MailMate with respect to this. MailMate cannot attach folders without the user manually compressing the folder first (e.g., using the “Compress” menu item in the Finder). This is actually what some other email clients do automatically since there is no other way to send a folder via email.

If it really is a single file, do you then have an example which you can share with me?

Sorry, wrote incorrectly. It's not a folder, but a budget file created in Movie Magic Budgeting. I can create a sample budget to send you, you wouldn't be able to open it because you don't have the program but maybe you'll be able to spot what the problem might be. Until these upgrades, I could always attach the files through Mailmate.
Thanks

Ok, some applications save what looks like a file but which really is a folder, but if it worked on Sierra then this is not the issue. You are very welcome to send me an example. I don't need to be able to open it.

I upgraded from a much older operating system, can't remember which but skipped Sierra.. But a file attached.

I don't seem to be able to reproduce the issue. Which method do you use to attach the file?

(Note that since it's not an image then it's not shown inline. It can only be seen in the Attachments popup in the status bar.)

what i've always done, just clicked on 'attachments' and gone to the file and clicked on attach.

couldn't get it to open through Terminal..

You need to quit MailMate first. Also make sure that I provided you with the path to MailMate that you are normally using. (You might have more than 1 version installed.)

still can't get it to open through Terminal...

And you get the same error message? (About the database lock.)