I just exported a project and saw the following prompt:
This prompt came for every element, including HTML files.
I’ve never seen this before. Also I have no idea what I did to cause this … (?)
You get this if you drag elements onto the blocs application main window.
duhhh 
Now I see what I did. Had my FTP program open, and dragged my exported files from Finder into my FTP program. What I didn’t pay attention to, was the fact that Blocs was open in the background, laying underneath my open FTP program.
Blocs tends to do this when you drag items over the top, even when it is not the active app and I’ve had this cause a few problems in the past. I am not convinced this is correct behaviour, so it is a good practice to keep the Blocs main app window in the dock while dragging other items around the desktop.
It’s not, I’m guna look into this. It’s only supposed to trigger on a file drop.
I can verify Flashmans post - was just able to reproduce it.
I have Blocs open, finder is opened on another part of the same screen, and my FTP program is open but on top of Blocs (active). If I drag a file from the finder over into the FTP program, Blocs shows the dialog overlay with the question “Are you sure …”