Keep a file in the archive

Keeps a file exactly as it came, with its SHA-256, in the Pulley archive (any file a provider exported), among cap table imports (a cap table export to import), or among other securities (a document for a security that isn't a SAFE, kept outside the register and put in round_id's binder). Pass the file as file_name and content_base64, up to 25 MB, or an uploaded document_id. A file already in the archive is refused. A spreadsheet's columns come back for safe.import.table.