Just do nothing and wait for the next virus-database update. If it is a false positive, it will be corrected quickly.
You may also put winmail.exe on the exclusion lists of the scanners (fileshield, behavior shield), but I would rather recommend to live temporarily with the "allow" clicking - it won't take long until the database is corrected.
But to be sure that your winmail.exe is not corrupted by a virus, upload "winmail.exe" on virustotal and have it analyzed.