This happened to me over the weekend too. My html files are more complicated, but the earliest (simplest) one I can find that screams "Virus warning!" when attaching to an email doesn't have any tabsets in it, just a kableExtra table and a DT table (and headers, graphs, table of contents). I wonder if it's something about the interactivity of the DT::datatable() tables that google is flagging?
Update - just made a couple of test files and it looks like it is the DT output that google doesn't like. I have no idea what to do about this though. Hoping someone else can pop in with some solutions (and/or a way to convince google that DT::datatable isn't malware)
