Since MTP read support went live in ChromeOS on the Stable channel in October last year, Google has been under some pressure to enable write access. Write access has apparently landed for users on the Dev Channel now, meaning those on the cutting edge can start writing data to their Android devices.
The update isn’t automatically live as soon as you update your ChromeOS device to the Dev channel, you will actually need to enable the “MTP write support” flag via the chrome://flags tab, where it lives. Once you’ve enabled the flag, restart your session and then plug in your phone or tablet and you’re good to go.
It’s pertinent to note that this is an experimental flag at this stage. There’s also some conditions, apparently image editing isn’t supported yet, but it’s a start. If you hit any issues report it to the Chrome team via the Chromium Bug Tracker or hit