The plot thickens! Apparently this is not a global issue, but something specific to my user account. Others on our JiveX do in fact see the attach option - so this appears to be something going on with the Settings on my specific account. I am a full admin on the site (but other admins are not having the same issue). So only thing I can guess is that it's a specific flag on my account, but I'm shooting the dark trying to find what that might be...
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Happy to say I was able to figure this one out!
In this case, the culprit was a User Override that had been placed on my account my someone at some unknown time. In Permissions -> Other Content Permissions, a "User Override" was on my account set only to "Customize Site". But apparently since all the other boxes weren't checked, it was keeping me from being able to add attachments. So once the override was removed, I can add attachments to content again.