Bike .bike files are really .html files with a different file extension. Unfortunately QuickLook doesn’t seem to understand this and so doesn’t generate previews. Eventually I need to figure out the problem, or maybe generate my own previews, but that’s low priority at the moment.
In the meantime you can get previews if you:
Change your .bike file extension to .html. Bike will still be able to read/write the file using that extension, and Finder will generate a preview since it’s a .html file extension.
The default HTML doesn’t look the best. There is some work in this thread to provide a better CSS preview and instructions for how to use it: Bike format Goals
Be careful, though, not to edit them in DEVONthink, which, by default, changes not only their file extension but also their internal format, leaving them unreadable to Bike.