On Apple Mail I get an error message "the attachments are still being loaded" - any fix?

I recently "upgraded" to OS X Sierra. All seemed OK, but I recently tried sending an attachment (from the desktop) with an email and got this:

No matter how long I wait (and for a 114kb attachment how long should it be? ) it never resolves.

The only thing I have done that is a bit different is turning off iCloud (I don't want to entrust my documents to third party, and need my computer to work WITHOUT an internet connection now and then, sorry Apple) but I wasn't sure if that was the reason for this. A quick online search got me a rather unhelpful tech support thread. I am hoping there is some quick-and-dirty way to fix this bug. The only reason I went to OSX Sierra was a piece of software that said I had to "upgrade" to use it. (Turbotax, as it happens, and no I am not entrusting anything of mine to their online service either).

Any ideas out there? The thought of wading through tech support will only raise my blood pressure.

ETA: Reboot seems to have solved it, but I'd still be curious if there's a cause.

Solution 1:

The Photos library repair tool resolved the problem for me. Hold down the Option and Command buttons while opening Photos, then select the repair option.