I'm fairly certain it can't be done without either a firmware update or unless the feature is patched in as an update to Astro.
Each app has its own version of a basic file explorer to locate files to open because default file associations have not been implemented into the android OS yet.
It seems like a pretty basic feature, i'm sure it won't be long before its added. On the app management screens there is a button for defaults but its always greyed out on my device.
Astro would need permissions to launch other apps when a certain file type is selected so i'm not sure if they will implement it that way for memory reasons, who knows.