Confluence should not allow attaching a file using drag and drop when that page is in view mode. Attachments should only be added when the page is in edit mode. The reasoning being that viewing a page doesn't mean a user intends to add something to the page.

4859f6618458 expanded on the issue in the comments:

Here is my view on the issue:

• The root problem is "accidental drops create unexpected/unintended attachments".
  • The solution that this issue suggests might not fit all users, i.e. attach without edit can be considered a feature!
  • As suggested earlier

    it is good practice to provide small drop zones for website-specific behaviour instead of the full page

• The root problem is a security/privacy issue; see my comment on CONFSERVER-40922 (that is supported by two unrelated users).
• You have fixed a subset when the source of dragging is on the page in CONFSERVER-75002.
  • Well, not exactly; see CONFSERVER-79237.
  • And the title of CONFSERVER-75002 should read "anything from the page [...]", because it did not fix it for just "anything" (that would include external content).