Hi Davo,
Thanks very much for your reply, I am not sure about this:
1. update the mdl_files record, so that, whilst the filename would appear unchanged, the content of the file (when downloaded) would be the content of the dummy file
If the filename remains the same would the file still open?
By that I mean if the original document was docx and we replace with a contenthash for PDF wouldnt the suffix / mime type both need to be updated in order for the browser to handle the file correctly?