Bridge deleted file extension, how to undo/restore?

Bridge usually makes it hard to remove the filename extension and you generally have to set it up to actually explicitly remove the extension.

You are best remembering to check the "Preserve current filename in XMP metadata" before renaming... Or renaming with the copy to other folder option so that the original filenames can be recovered.

Can you share a screenshot of your Batch Rename settings that caused this issue?

If all of the images were the same file format (all TIFF, or all JPEG etc), then you can batch rename as illustrated here:

Otherwise, you may have to work out which files are the same format and rename in smaller groups with the correct extension.

If you have a large group of files which are all different filetypes, then I would recommend using the CLI based ExifTool program to automatically work out which is which:

https://prepression.blogspot.com/2017/09/exiftool-automatically-adding-missing.html