Problem with Dropbox .skp files on iOS?

I copy .skp files to my Dropbox folder on my main computer.

That syncs them to Dropbox “in the cloud”.

They show in a browser, with their proper file size - eg 6.7MB.

On both my iPhone 12 and iPad Mini, when I view them in Chrome browser on and download to my phone or iPad, the downloaded file is only 833 bytes, and when I try to open it in SU Viewer (on phone), or SU beta iPad app I get the message “Oh Deer” (sic) “this doesn’t appear to be a valid SketchUp file”.

Other file types (eg PDFs or image files) download and open properly on both iOS devices.

Any ideas on how to get this to work properly? Or even what’s causing this issue?