--- title: contentType description: A security guard that decides each upload's Content-Type from its bytes, not the client's claim. Magic-byte sniffing stops a mislabeled .png that's really HTML or SVG from being stored as an image and served inline. No metadata, no native dependencies, and it never buffers the whole body. --- The built-in `contentType()` plugin sets an upload's `Content-Type` from what the bytes **actually are**, not what the client said they were. On `upload` it magic-byte-sniffs the body and either corrects the stored type to match (the default) or rejects a mismatch — so a `.png` whose bytes are really HTML or SVG can't be stored under an image type and later served inline. It's the [wrap plugin](/plugins#wrap-intercepting-operations) counterpart to [`validation()`](/plugins/validation): validation vets the _declared_ type, `contentType()` verifies the _real_ one. ```ts lineNumbers import { createFiles } from "files-sdk"; import { s3 } from "files-sdk/s3"; import { contentType } from "files-sdk/content-type"; const files = createFiles({ adapter: s3({ bucket: "uploads" }), plugins: [contentType({ onMismatch: "reject" })], }); await files.upload("avatar.png", pngBytes); // ok — bytes are a PNG await files.upload("avatar.png", htmlBytes); // throws — bytes are HTML ``` ## What it recognizes The sniffer is deliberately scoped to where a verdict is unambiguous and useful: - **Images** — PNG, JPEG, GIF, BMP, WebP, TIFF, and ICO, by their magic bytes. - **PDF** — the `%PDF` signature. - **HTML, SVG, and XML** — the security-relevant part. These are text, so they have no fixed magic bytes; a leading text scan (skipping a BOM and whitespace) catches ``, ``, `