Hello!
Actually, last time I posted from the PC version (full version), I faced the same problem :(
However, I remember that before I was able to upload the image and then click on it to paste to the place that I needed).
The Mobile interface allows you to have such functionality (I mean user interface) - attachments are listed at the bottom and they can be clicked to be added (it can be done). But the text editor in the mobile view (and HiKnow app) isn't 'Advanced'.
The solution could be to add the BBCode of the attachment in the mobile view when clicking on the image. In this case, the user will see that the attachment is ready to be insert wherever they need to insert it and the text editor can still be the simple text editor.