@jestemzero
Two really important factors I took into considering when building this mechanism is to keep it TZIP compliant and to ensure this will not break anything on marketplaces, wallet software, indexers, etc. The use of unique metadata keys is compliant and friendly all around. Keeping the portfolio index outside of the metadata simplifies our metadata and never needs to be seen by indexers and marketplaces if they do not care to include that information. End users can still see the artwork files and retrieve the assets they purchase even if the Name and Description aren't shared on the marketplace.
And even when the filetypes stored in the extraUri metadata are a format not recognized by the marketplace, this will have zero negative affect because we preserve the integrity of the other artwork strings marketplaces already use (artifactUri, thumbnailUri, externalUri). Marketplaces can decide for themselves if and what they wish to display without artists worrying about their listings looking broken.