[Original Article: 10.4: Disable Spotlight on a FAT32 external drive • comment by: dickg]
Although this is an old thread, I’d like to point out there may be a distinction between a filename and a “file extension”. The stand-alone file named “.metadata_never_index” is a hidden file with the filename of “metadata_never_index”. But something like “banana.metadata_never_index” is a visible file (or directory or volume) with a “file extension” of metadata_never_index. Spotlight looks for the invisible file with the filename of metadata_never_index. As a file extension, it could have another adverse effects, and probably should be avoid …

Source: Mac OSX Hints