The endpoint they delivered doesn't do what it promises: POST /api/v3/workspaces/ {ws} /custom_fields/ {field} /attachments returns HTTP 500 “context deadline exceeded” for any file over 15-20 MB, and the documented attachment limit is 1 GB. I tested Workspace Business Plus on 25/08, from two different networks (office and a VPS in a datacenter): 15 MB goes up, 20, 30, 60, 120 and 300 MB all fail, always about 3 seconds after the body finishes climbing. That is, it's not a connection timeout on my end, it's the backend of you giving up processing the file. And there is no outline through the API: the same file goes up normally through the interface and as a task attachment through v2 (I tested up to 600 MB), but the task attachment is rejected as a field value (FIELD_250) and is not uploaded via a signed URL to the field. In practice, for any real media file, especially video, the functionality does not exist via API. Provide visibility of the correction somewhere. Automation that needs to fill in a file field with rendering or video delivery is blocked until that endpoint respects the 1 GB limit.