A Heur. detection means a detection by the heuristic calculations of the AV. This is an extra way of detecting malware but in CIS can be prone to false positives. To check whether it is a false positive upload it to both Virus Total and CIMA.
Heuristic definitions are an attempt to identify malware that nobody has yet seen.
In the case of Heuristic.Dual.Extensions these are files that have a name like example.exe.jpg to fool the user into running them. They can also be legitimate files that are temp files that will later be renamed for their purpose.
For the future you should probably only quarantine the files and not delete them. Also, if you’re not sure that a file identified as malicious actually is you can report it as a false positive here. They will analyze it and send you an email letting you know if it is actually dangerous or not.
You have to remove it from quarantine in order to upload it. Of course if you want you can make it a blocked file using CIS so it can’t do anything anyway, but you can upload it.
actually that does not look like a false positive to me, especially seeing as sophos and dr.web report it as malware and they almost never have them. I would keep it in quarantine and if everything is running good in a few days I would delete it.