Marta here, from Trello Support.
Right now, only when you upload image files to a Trello card, Trello will automatically create a picture to put on the front of the card and in the header of the card back. These are called "card covers." Card covers aren't available for image files that are linked via Google Drive, Dropbox, Box or OneDrive tough. They're also not available for other types of files, like documents or spreadsheets.
That being said, I'll pass your interest on that to my team so they can take it into account.
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