Upload a table attachment

Stage a file for a files field, or — with no fieldKey — for a message on a row's thread. The request body is the raw file; its name and target field ride the query string. Files must be 25MB or smaller and of an allowed type (images, PDF, CSV, plain text, common Office documents) — anything else, including SVG, is refused outright rather than stored and relabelled later. The upload is STAGED: bind a cell file by PUTting the cell's complete membership to /{tableId}/rows/{rowId}/attachments/{fieldKey}, and a thread file by naming it in attachmentIds on POST /{tableId}/rows/{rowId}/messages. An upload never bound is destroyed automatically after 24 hours.