File masking

Prompts are not the only way data reaches a model. Files uploaded for retrieval, attached to a message or embedded as base64 often carry far more sensitive content than the prompt itself. Shadow masks them before they leave the machine.

Where files are found

Transport	Examples
`multipart/form-data` uploads	OpenAI `POST /v1/files`, `POST /v1/uploads` parts
Base64 files inside JSON	OpenAI `input_file.file_data`, Anthropic `document` blocks with a `base64` source, Gemini `inline_data`
`data:` URIs	`data:text/csv;base64,...` anywhere in a JSON string

The format is recognised from the declared MIME type and the file name.

Formats

Format	Treatment	Result
Plain text, Markdown	Masked as text	Same file, masked
CSV	Masked as text, cell by cell	Same columns and rows
JSON	Masked as JSON (string values)	Valid JSON
HTML, XML	Masked as text	Markup intact
DOCX	Masked in place: the document’s text runs	A valid Word document
XLSX	Masked in place: shared strings	A valid workbook; numbers and formulas untouched
PPTX	Masked in place: text runs on slides	A valid presentation
PDF	Text is extracted, masked and sent as text (`pdf = "text"`)	The model receives the masked text instead of the PDF
Images	Passed through unchanged by default (`images = "pass"`)	No OCR yet
Anything else	Passed through

Masking uses the same detectors, persona, vault and intent policy as prompts, so a customer named in the prompt and in an attached spreadsheet gets the same fake in both. A customer list attached to a report request gets typed tokens.

Office documents are edited inside their ZIP container, so styles, tables and layout survive. A value split across differently formatted runs (for example a name that is half bold) may not be recognised.

PDF options

PDFs cannot be safely edited in place, so Shadow offers three choices:

`pdf =`	Behaviour
`"text"` (default)	Extract the text, mask it, send it as a text document. Layout, images and scanned pages are lost
`"block"`	Refuse the request with an error, so a PDF never leaves unmasked
`"pass"`	Send the PDF unchanged (not masked)

Images

Shadow does not run OCR yet. Images, including screenshots and scanned documents, are passed through unchanged by default. If images may contain sensitive data, block them:

[files]
images = "block"

Configuration

[files]
enabled = true          # mask uploaded files (default true)
pdf = "text"            # text | block | pass
images = "pass"         # pass | block
max_bytes = 52428800    # 50 MiB; larger files are not masked

Limitations

No OCR. Text inside images and scanned PDFs is not detected.
PDF layout is not preserved in text mode. Questions about the visual layout of a PDF will get worse answers.
Files above max_bytes are forwarded unmasked with a warning, or refused when block_oversize = true.
Uploads by reference. A file already uploaded to the provider earlier (by file_id) is not re-scanned. Upload files through Shadow from the start.
Embedded objects in Office files (charts’ cached values, embedded workbooks, images) are not masked.
Archives (.zip, .tar) other than Office formats are passed through.

Check what Shadow finds in a document’s text with shadow scan on an exported text version before you rely on it.