We manage GIS for several dozens municipal public works departments and the bulk of attachments we handle are engineering plans, as-builts, and survey maps. It is a mix largely composed of PDF and TIF, JPG and PNG to a much smaller extent, that these communities have. TIF is a very common format that we encounter.
Historically, we have used our own storage and external web applications for handling attachments for our clients, but are migrating everything into hosted feature layers, as per licensing and MSP requirements.
