I think we could check whether the drag item is a OS file folder here: https://searchfox.org/comm-central/rev/e4cbcd9179fa7ebeed2025eafbd2ec27f8a7d066/mail/components/compose/content/MsgComposeCommands.js#9839-9840
Bug 188598 Comment 15 Edit History
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I think we could check whether the drag item is an OS file folder here: https://searchfox.org/comm-central/rev/e4cbcd9179fa7ebeed2025eafbd2ec27f8a7d066/mail/components/compose/content/MsgComposeCommands.js#9839-9840