You can find whether the files are owned by a package:
pacman -Qo /usr/share/icons/oxygen/64x64/mimetypes/fonts-package.png
If they are not owned by a package, you can decide whether to force an overwrite with pacman, or just delete the files from disk.
Also, web search "pacman exists in filesystem" would get you similar answers.
Edit: or just do what oberon said.
