feat: [DLCM-2434] determine file type and add deduced extension for Thumbnail and DUA
Test it with: solidify/solidify-backend!381 (merged)
And frontend: dlcm/ui/dlcm-portal!575 (merged)
Test it with: solidify/solidify-backend!381 (merged)
And frontend: dlcm/ui/dlcm-portal!575 (merged)