Just saved a test render of an older file in TG3.4 beta and it saved automatically in .bmp...thought that had changed to .tif in this version or did it default t the older version.
If you're just saving a render from the Render View, it will save with the format you choose in the file save dialog. This defaults to whatever you have set in Preferences: File Saving. In your case it's probably still set to BMP. The 3.4 update won't change this setting for you. I recommend changing this to TIFF 8-bit-LZW as it saves a lot of disk space compared to BMP.
The new default setting mentioned in the change log only affects the settings on the render node's "Sequence/Output" tab ("Output image filename" and "Extra output images") which are used by the buttons "Render All To Disk" and "Render Sequence". Also, the only time this default matters is when TG initially sets up these filenames when creating a new project or new render node, so it won't change an existing project that you loaded.