This caused me much grief! So I thought I’d share how I got it working.
If you need to create a webflow cms item with a dropbox file, the standard dropbox share links are not a direct link to the file. After creation of the dropbox file, I found I had to use a dropbox api call module to the [get_temporary_link] (https://www.dropbox.com/developers/documentation/http/documentation#files-get_temporary_link) endpoint.
I also found I had to use a Webflow api call module to create the item. I originally tried to use the dropbox download file and the webflow create item modules with no luck.
I hope this helps others preserve some hair in the future!