# FileTypeValidator

**Kind:** Class

**Source:** [`packages/common/pipes/file/file-type.validator.ts`](https://github.com/nestjs/nest/blob/master/packages/common/pipes/file/file-type.validator.ts#L78)

**Part of:** [Common](subsystem-packages-common)

Defines the built-in FileTypeValidator. It validates incoming files by examining
their magic numbers using the file-type package, providing more reliable file type validation
than just checking the mimetype string.

`FileTypeValidator` is a built-in file validation class used with NestJS file upload pipes. It inspects the uploaded file buffer for magic numbers via the `file-type` package, making validation more reliable than trusting the client-provided MIME type alone.

**Extends:** `FileValidator`

## Methods

| Method | Signature | Returns |
|---|---|---|
| `buildErrorMessage` | `buildErrorMessage(file: IFile)` | `string` |
| `isValid` | `isValid(file: IFile)` | `Promise<boolean>` |

## Where it refuses work

- `FileTypeValidator` stops the work with an early return when `this.validationOptions.fallbackToMimetype`, in 4 places.
- `FileTypeValidator` stops the work with an early return when `errorMessage`.
- `FileTypeValidator` stops the work with an early return when `file?.mimetype && !file.buffer && !this.validationOptions?.fallbackToMimetype && !this.va…`.
- `FileTypeValidator` stops the work with an early return when `!this.validationOptions`.
- `FileTypeValidator` stops the work with an early return when `this.validationOptions.skipMagicNumbersValidation`.
- `FileTypeValidator` stops the work with an early return when `!isFileValid`.

## When something fails

- `FileTypeValidator` handles failure in 2 places: it logs it and continues in 1, and turns it into a return value in 1.

## Diagram

```mermaid
graph LR
  A[Uploaded file] --> B[ParseFilePipe]
  B --> C[FileTypeValidator]
  C --> D[Inspect file buffer magic numbers]
  D --> E[file-type detects MIME type]
  E --> F{Matches configured fileType?}
  F -->|Yes| G[Accept file]
  F -->|No| H[Return validation error]
```

## Usage

```ts
import {
  Controller,
  HttpStatus,
  ParseFilePipe,
  Post,
  UploadedFile,
  UseInterceptors,
  FileTypeValidator,
} from '@nestjs/common';
import { FileInterceptor } from '@nestjs/platform-express';

@Controller('uploads')
export class UploadController {
  @Post('image')
  @UseInterceptors(FileInterceptor('file'))
  uploadImage(
    @UploadedFile(
      new ParseFilePipe({
        validators: [
          new FileTypeValidator({
            fileType: /image\/(jpeg|png)/,
          }),
        ],
        errorHttpStatusCode: HttpStatus.UNPROCESSABLE_ENTITY,
      }),
    )
    file: Express.Multer.File,
  ) {
    return {
      filename: file.originalname,
      detectedType: file.mimetype,
    };
  }
}
```

## AI Coding Instructions

- Use `FileTypeValidator` inside `ParseFilePipe` validator lists for uploaded files handled by Multer interceptors.
- Configure `fileType` with either an exact MIME type string, such as `image/jpeg`, or a regular expression for multiple allowed types.
- Do not rely only on `file.mimetype`; this validator checks the file buffer contents and is intended to prevent spoofed MIME types.
- Ensure the upload adapter provides a `buffer` on the uploaded file, since magic-number detection requires access to file contents.
- Combine this validator with `MaxFileSizeValidator` when endpoints need both type and size restrictions.