Chromium – Open downloaded files

If you wonder, how chromium decides, with which program it opens a downloaded file, take a look at xdg-open

To change the behavior you have to edit ~/.local/share/applications/mimeapps.list

You can also do that, by using a nice tool. For example, if you want to let chromium open pdf files with evince try this
xdg-mime default evince.desktop application/pdf