Why am I getting a 31 character limit on file saves from Mail all of a sudden? Is this because I installed macOS Sierra from Yosemite a couple of months ago, it's on my B Mac so don't use it so much.
From the screenshot I am assuming you're trying to save an attachment directly from Mail.

If so, there is a known issue that a corrupted Mail index causes this odd 31 character file size limit when saving from Apple Mail. The best way to correct this is to follow Apple's instructions for rebuilding your Spotlight index.

If my assumption is wrong (re the Mail app), let me know and I'll update my answer with broader solutions beyond just the Mail app issue.