We overhauled our website not too long ago and all the existing links were broken.
Here is the general page that talks about File Format for an IGDB submission, which is the same format that Optics needs.
On that page there are links to example files for different glass types.
Here is a link to the specific document that describes the format in detail.
These files need to be text files, and I would think you might want to just use a txt extension.
As far as I know a png file is a graphics file format, but maybe there is another use of that file format that I don't know about.
Robin