Why? Many people use 3rd party software to manage S3 stored content. An example is the popular desktop Mac application 'Cyberduck' - https://cyberduck.io/. The application is not a previewer, but it does include a feature, 'quick look'. Desktop though.
Here's what a quick search turned up, related -
It's likely that sophisticated users already have a CMS, or use a CDN, to provide preview. Would a web application that uses PrizmDoc S3 capabilities, PrizmDoc Cloud (Accusoft Hosted), and the viewer make sense to you at all Mark? It's been a long week, and maybe I am not thinking this through all the way. Of course we already do this, and have this capability, but I'm thinking of packaging and marketing a specific feature, 'preview your S3 objects, no code required'.
The S3 API is no longer unique to AWS. We use it, and it seems that other disruptors are taking advantage of S3 expertise in the dev community, for example - https://wasabi.com/. 3rd parties - https://wasabi-support.zendesk.com/hc/en-us/sections/115000379791-Third-Party-Storage-Applications.
This is a startup by the same guys who built carbonite, among many other things. Flowers and Friend. - https://wasabi.com/about/.
Here's another - https://knowledge.hitachivantara.com/Documents/Storage/Content_Platform/8.2/Developer_documentation/Using_the_Hitachi_API_for_Amazon_S3/13_Using_third-party_tools_with_the_Hitachi_API_for_Amazon_S3.