Hi Gabriel! It’s true that people would usually ask for a file type by extension (.PSD or .AI), however we found that the brand color and the 2-letter product logo were a quicker and easier way for users to immediately recognize a primary file type. Especially if you’re looking in List view in the Finder window and the icon size is at 16px. When the icon is that small, we drop the file type mime anyway (in both the old and new version of the icon), so it was more important for us to feature the two-letter logo more prominently for those smaller sizes.
Another reason for moving the filetype mime down to the center of the icon was that it allowed us to keep this visual element in a consistent place across all the surface types (document, pad, book, envelope, etc.). With our old tag design, the placement of the mime would vary depending on the icon surface, and that would have made production much harder to automate when it comes to scripting in Illustrator.