dotnet/aspnetcore · error

FileExtensions validator requires a non-empty "extensions" p

Error message

FileExtensions validator requires a non-empty "extensions" parameter.

What it means

Thrown by the FileExtensions validator (FileExtensions.ts:12) when params.extensions is falsy. This validator checks that an uploaded filename ends with one of the allowed extensions (e.g., '.png,.jpg'); without the extensions list there is nothing to validate, so it throws to surface the misconfiguration.

Source

Thrown at src/Components/Web.JS/src/Validation/Validators/FileExtensions.ts:12

// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

import { ValidationContext, ValidationResult, Validator, pass, fail } from '../ValidationTypes';

// Validates that the filename ends with an allowed extension (case-insensitive).
// Extensions param is comma-separated with dot prefix (e.g. ".png,.jpg,.gif").
// Throws if the mandatory `extensions` parameter is missing.
export const fileExtensionsValidator: Validator = (context: ValidationContext): ValidationResult => {
  const { value, params } = context;
  if (!params.extensions) {
    throw new Error('FileExtensions validator requires a non-empty "extensions" parameter.');
  }

  if (!value) {
    return pass();
  }

  // Build regex from comma-separated extensions, stripping dots and escaping regex metacharacters.
  const extensions = params.extensions.split(',')
    .map(ext => ext.trim().replace(/^\./, ''))
    .filter(ext => ext.length > 0)
    .map(ext => ext.replace(/[.*+?^${}()|[\]\\]/g, '\\$&'))
    .join('|');

  if (!extensions) {
    return pass();
  }

  return new RegExp(`\\.(${extensions})$`, 'i').test(value) ? pass() : fail();

View on GitHub (pinned to 294cab2f9b)

Solutions

1. Add [FileExtensions(Extensions = ".png,.jpg,.gif")] to the IFormFile/string property and confirm the input renders data-val-fileextensions-extensions=".png,.jpg,.gif".
2. When wiring manually, pass params: { extensions: '.png,.jpg' } (comma-separated, leading dots optional).
3. Confirm no custom HTML helper is stripping data-val-* attributes from file inputs.
4. Verify the attribute is on the property bound to the uploaded file, not a related display field.

Example fix

// before
<input type="file" name="Upload" data-val-fileextensions />

// after
<input type="file" name="Upload" data-val-fileextensions data-val-fileextensions-extensions=".png,.jpg" />

function hasExtensionsParam(params: Record<string, unknown>): boolean {
  return Boolean(params && typeof params.extensions === 'string' && params.extensions.length > 0);
}

function isFileExtensionsConfigured(p: any): p is { extensions: string } {
  return p && typeof p.extensions === 'string' && p.extensions.trim().length > 0;
}

When it happens

Trigger: Invoking the fileExtensions validator with a params object missing 'extensions' or with extensions='' / null. Normally emitted from [FileExtensions(Extensions=".png,.jpg")]; if the attribute's data-val-fileextensions-extensions metadata is absent the JS validator cannot run.

Common situations: Model has [FileExtensions] but the attribute argument is empty or the tag helper does not emit data-val-fileextensions-extensions. Custom input rendering that drops data-* attributes. Manually registering the validator without supplying extensions.

Related errors

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• regex validator requires a non-empty "pattern" parameter.
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• Unknown passkey operation '${this.attrs.operation}'.

AI-assisted analysis of dotnet/aspnetcore@294cab2f9b (2026-08-06). Data as JSON: /api/errors/ab50cb4ecc602f87. Report an issue: GitHub.