flashplugin package can’t be used for Chromium browser on Arch Linux. Use chromium-pepper-flash from AUR instead.

$ wget https://aur.archlinux.org/packages/ch/chromium-pepper-flash/chromium-pepper-flash.tar.gz
$ tar zvxf chromium-pepper-flash.tar.gz
$ cd chromium-pepper-flash
$ makepkg
$ sudo pacman -U chromium-pepper-flash-1\:16.0.0.235-3-i686.pkg.tar.xz
$ chromium &