Validation & limits

Declare limits as data — at the module level (applies to every route) and/or per route (merged over the module default). Rejections throw typed 400s before your handler runs.

// module-wide default
NestFileStorageModule.forRoot({
  default: 'local',
  drivers: { /* ... */ },
  validation: { maxSize: 10 * 1024 * 1024 }, // 10 MB everywhere
});

// per route (merged over the module default)
FileStorageInterceptor('image', {
  validation: {
    maxSize: 5 * 1024 * 1024,                                 // bytes
    allowedMimeTypes: ['image/jpeg', 'image/png', 'image/*'], // wildcards supported
    allowedExtensions: ['.jpg', '.png'],                      // case-insensitive
    maxFiles: 10,                                             // total files per request
    fileFilter: (req, file, cb) => cb(null, true),           // escape hatch (runs last)
  },
});

What each rule does

Rule	Becomes	Throws on violation
maxSize	Multer limits.fileSize	FileTooLargeException
maxFiles	Multer limits.files	TooManyFilesException
allowedMimeTypes / allowedExtensions	a generated fileFilter	InvalidFileTypeException

All three exceptions extend BadRequestException, so they surface as standard 400 responses with a clear message:

{ "message": "File type \"text/plain\" is not allowed. Allowed: image/jpeg, image/png.", "error": "Bad Request", "statusCode": 400 }

{ "message": "File exceeds the maximum allowed size of 5 MB.", "error": "Bad Request", "statusCode": 400 }

MIME wildcards & extensions

• allowedMimeTypes matches exact (image/png) or wildcard (image/*, */*).
• allowedExtensions is case-insensitive and accepts entries with or without a leading dot (png or .png).

Cross-field rules

For rules that depend on multiple files/fields together, use afterUpload — e.g. "at least 2 images", "a cover is required when gallery is present".

:::tip Don't validate inside fileName In v1 it was common to throw from fileName(). Use declarative validation instead — it runs at the right time (size is enforced during streaming) and returns typed errors. :::