I just realized by '/folder_contents' you meant TTW
- that worked with the images and files
- interestingly now the text files (html / rst) only registered with header info - no body info
- folder hierarchy got flattened.
Thanks for your responses and suggestions though - much appreciated!
TTW upload of text documents (html / txt / rst) issues mentioned here by the way - as far as I know not yet resolved.
https://community.plone.org/t/bug-upload-text-file-to-folder-nothing-there-plone-5-1-rc-2/5609