# Files in and out

- A file never passes through you, in either direction: no tool takes a file's bytes or text, and none gives them back.
- A file comes in by upload alone, named by its upload_id in the tool that files it (books.attach_receipt, books.file_document, books.set_w9 w9_upload_id, books.import_bank_csv, books.import_chart_csv): books.begin_upload with the file's sha256 (`sha256sum`), size (up to 25 MiB), type, name and purpose; run the command it returns (`wanderbooks upload`) with the file's path; then give the upload_id. An upload lasts 15 minutes, is used once, and only by the token that began it.
- If begin_upload says the bytes are stored already (existing_attachment_id), link that attachment instead of filing a copy.
- A file goes out by download alone. Files the books make (books.export in every format, books.asset_labels, books.export_cpa_package, books.vendor_1099 csv, books.get_w9) are handed over: run the download command each returns to save it in a folder. They are kept 15 minutes, for the token that asked. No file with a full TIN goes into the books, however it is filed: the server refuses every file it read or wrote with full TINs (the W-9 books.set_w9 took, and each of those outputs, or a file taken out of a CPA package), and any text file holding one: a TIN the vault keeps, written as nine digits or in the usual groups (3-2-4 with hyphens, spaces or dots; 2-7 with a hyphen or a space), or a number printed as an SSN or EIN is (3-2-4 or 2-7 digits joined by hyphens, not part of a longer number). Other numbers pass, so bank exports (routing and account numbers, long references) go in. It can't recognize an altered copy of a scan, or a TIN the vault doesn't keep written any other way, so never file a W-9 or a TIN worksheet yourself.
- Receipts and documents come back out through books.get_receipt view bytes, handed over the same way: run its download command to save a copy in a folder you name.