Data Transfers · Setting up a transfer
Why can I only pick folders, not single files, with the folder button?
Short answer: The folder button picks folders because transfers run between locations. To send one file, put it in its own folder and transfer that; for a file on the web, use Download with its link.
The folder button chooses folders, because a transfer is set up between two locations. To send one file, the simplest way is to put it in a folder of its own and transfer that folder.
If the file is on the web, use Download with its link instead — that operation is built for a single file and can resume large ones.
Related: Download a file from a link · Start a new transfer