If you wish to prevent certain filename extensions from being created during Safe Copy, which creates SBSE___.XXX (XXX is random) temporary files as part of its two-stage copy process, you can do so by adding such extensions into a special text file. SyncBack will then know to exclude such file extensions when it generates a random Safe Copy extension during such operations.
For example, some AV programs may flag & block files with certain extensions when they are being created/accessed as a "known bad file". Adding these extensions into the list will prevent them from being used as a temporary filename in future profile runs.
Create a text file called blockext.txt in the folder where the profile settings are stored. For example:
C:\Users\username\AppData\Local\2BrightSparks\SyncBack
Depending on the product installed, adjust the final folder, e.g. SyncBackPro if using the Pro version. This article has more details on where your profile settings are stored:
https://help.2brightsparks.com/support/solutions/articles/43000335836
Then add filename extensions that you DO NOT want SyncBack to use with Safe Copy filenames into this file. For example,
KUW
ABC
XYZ
Do not include the period (.) before the text and only have one extension per line. Only include three-letter extensions here. Anything other than three-letter extensions like XX or XXXX will be ignored.