The file comes from wherever you have it stored in the first place (FTP, S3, SFTP, virtually anything) and viewer are plugins implementing various interfaces which are packaged up onto a zip file. With this approach, to add support for psd, I just implemented the image viewer interface (https://github.com/mickael-kerjean/filestash/blob/a605988d5c...), to add support for parquet files I implemented the table viewer interface (https://github.com/mickael-kerjean/filestash/blob/a605988d5c...) and still have the freedom for custom stuff like this docx editor based of libre office wasm: https://github.com/mickael-kerjean/filestash/tree/master/ser... Most of the actual implementations of the base interfaces are done in C, compiled to wasm and running via a web worker