Users encounter error sending private messages with attachments

This only happens with private messages and non administrators (those with admin role do not encounter error). It DOES NOT happen when adding messages to forum posts, media galleries, etc.

Here is the error.

Only Known URLS and the following file extentions are allowed: (lists variety of file types from config). please select a valid file or enter a valid url.

We are on v10.1.0.7092