To configure which of these file types should be handled by BetterZip by default, e.g., when you double-click a file in Finder, go to BetterZip » Preferences » File Types and select the types to handle.
To add your own file extensions to this list, scroll to the last entry in the list named “User-Defined Archive” and click the field in the column “Extensions”. Then enter the extensions without a dot, separated by commas.
Some file types use an archive format (mostly zip) to store their data and only have a different filename extension. When opening such files BetterZip will recognize them as archives and show you their contents. In most cases you don’t want this, but rather have them treated like normal files. This is where the file types blacklist comes in handy: Add whichever files you want treated as normal files to this list. By default the list contains: pptx,docx,xlsx,numbers,pages,pdf,swf.