[iamcalledrob]

It used to be the other way round. Sheet of paper indicates document/file, arbitrary shapes for applications.

I'd argue that differentiating apps from each other is more important than file types apart. Probably

[internet2000]

Mac OS and iOS thumbnails documents/files for icons on the file browser and search, so shapes are irregular. E.g. Landscape thumb for a .pptx, square album art for an .mp3, portrait for a .pdf, arbitrary shape for a .xlsx. 3rd party apps can participate in that through Quicklook plugins.

It's a great feature because I can scan several files named export (n).xlsx on my downloads folder stack, and know which one I want from the thumbnail alone. OS feature improvements change the design context.