Thank you for your answer.
I already had assumed that there is no current version for Chrome OS. Considering that Google has made it possible to install Android apps in Chromebooks, it would have been silly to support a separate version for Chrome OS. However, Google has made it possible for Android app developers to support their apps in Chromebooks better, including providing access to the add-on storage such as micro-SD cards. The Chrome-OS does not extend this capability universally to all Android apps, its app much make a few changes to provide this updated support.
Considering that many of us receive a good number of attachments, I think that it would be a stellar idea for Aqua Mail to support microSD cards and other extended storage options such as SSDs. Minor changes need to made to the app to provide this support in Chrome.