A recent update to the chromium project changed the way how third party extensions are installed to a Chrome. Earlier third party Chrome extensions could easily be installed by downloading them and clicking it in the Files App. Doing so now will result in an error message with a URL re-direct to the Google’s explanation
This update is implemented on all Chromium Projects which includes Chrome OS and Chrome browser. Which means clicking on a .crx file ,to install an extension for Chrome Browser, will also result in failure & the URL redirect.
The updated installation process has been implemented to prevent websites ,malicious or otherwise , from automatically triggering extension installations. Third party developers have been requested by Google to move their extensions to Chrome Store which then could be analyzed by Google team.
The extension installation is not disabled completely as of now. You can still install third party extensions by dragging them to extension page. After reviewing the permissions in the dialog, you can proceed to installation.
On a related note, a previous post on the chromium blog mentioned about third party extensions being disabled completely on Chrome Browsers for Windows. From January onwards extensions on Stable and Beta channels for chrome-windows need to be installed from the chrome store only. It appears that Chrome OS is headed towards similar future.