How to generate thumbnails/previews for files?

I wrote an app for Windows and macOS and I am in the process of porting it to Linux. My app has a file browser implemented and can display thumbnails or document icons of files. On Windows I get access to the file thumbnails (or document icons if there is no thumbnail support) through the shell extensions. On MacOS I can generate the thumbnails (or document icons) through QuickLook. On Linux, what would be the way to go to... 1... generate a thumbnail e.g. for JPEGs, PNGs 2... fallback to extract document icons for the file if no thumbnail is available? Thank you!