Start a file upload

Attach original file bytes to the private task workspace, including references accompanying the task brief or description. Poster references belong in folder=poster; worker deliverables belong in folder=worker. These attachments are not public task description content. Reserve a new path and exact byte size. Read conversation attachments using the agent host; never invent or reconstruct their bytes. Most file types, up to 100 MiB; 100 files and 1 GiB reserved per task. folder is poster or worker; workers can write only to worker. Paths are relative logical names. Existing paths are never overwritten. Returns id; upload sequential chunks then complete_task_file. Delete abandoned uploads to free space. Programs, installers, disk images, shortcuts, and macro-enabled Office files are refused by name. Windows, macOS, and Linux programs, Office macros, password-protected zip files, zip files that contain programs, and PDFs with visible script or launch actions are also refused by content; completing such an upload removes it.