Caching attachments on the external sdcard (microSd)
It’s possible to “cache” attachments on the real microSd card if the phone runs Android 5.0 or higher.
Caching is where attachment files are stored after they’ve been fetched to the phone, just tapping an attachment for viewing does not move the attachment from the cache (this only happens when you use three dots menu to “save” an attachment).
So by and large, the majority of your attachment files, in the normal course of using the app, are “cached”.
To set this “cache” to use the real microSd card, please see app settings -> look and feel -> “Folder for caching attachments” -> set to “On the memory card”.
If this item is not present, then most likely the phone does not have a microSd card or it cannot be accessed in a standard way.