I have discovered a way in which I can attach documents stored in iCloud as links rather than attachments on iOS. It is rather clunky, requiring multiple steps:
1. Open Files app and browse to the folder containing target document.
2. Touch "Select" at the top right of the screen..
3. Select the desired document.
4. Touch "Share" at the bottom left of the folder window.
5. Touch the "Add People" share option.
6. Touch "Copy Link"
7. Another window open, touch "copy link" again.
8. Switch to Trello
9. Open the target card
10. Touch the attachment paperclip at the bottom right, which offers the option to "Attach copied link.
11. Once the link has been created it can be renamed.
The advantage of this is that activating the link opens the document in the appropriate app. For Apple Works documents, activating the link in Safari on macOS opens the document in the iCloud version of the app.
I haven't tested extensively, but this process doesn't seem to create any issues. I haven't done much testing with non-Apple Works document types except PDF. on iOS, the PDF is opened by Files, but on macOS, the document doesn't open--just the file browser.
Despite the clunkiness of the process, I think it will be very useful to be able to access the live versions of Works document from Trello.
So my questions are:
1. Is there a quicker way to get to the iCloud link?
2. Is there some hidden snafu that may cause problems in the future?