Personally, I won't use the plugin because I have an Antivirus program and I know when I'm safe or not to execute a received file of my contacts.
However the idea is good. Messenger shows (can show) a popup (I think) when you're gonna open an exe received file, but your plugin could add more security to that. It should work similar to what (for example) Norton Antivirus works with Win Messenger 4.7 (it scans just received files and in case it finds a virus ir sends an instant message to the contact who sent that file with a warning).
Well, as I said the idea is good. Go with it
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