By providing `validator` prop you can specify custom validation for files.

The value must be a function that accepts File object and returns null if file should be accepted or error object/array of error objects if file should be rejected.

```jsx harmony
import React from 'react';
import {useDropzone} from 'react-dropzone';

const maxLength = 20;

function nameLengthValidator(file) {
  if (file.name.length > maxLength) {
    return {
      code: "name-too-large",
      message: `Name is larger than ${maxLength} characters`
    };
  }

  return null
}

function CustomValidation(props) {
  const {
    acceptedFiles,
    fileRejections,
    getRootProps,
    getInputProps
  } = useDropzone({
    validator: nameLengthValidator
  });

  const acceptedFileItems = acceptedFiles.map(file => (
    <li key={file.path}>
      {file.path} - {file.size} bytes
    </li>
  ));

  const fileRejectionItems = fileRejections.map(({ file, errors }) => (
    <li key={file.path}>
      {file.path} - {file.size} bytes
      <ul>
        {errors.map(e => (
          <li key={e.code}>{e.message}</li>
        ))}
      </ul>
    </li>
  ));

  return (
    <section className="container">
      <div {...getRootProps({ className: 'dropzone' })}>
        <input {...getInputProps()} />
        <p>Drag 'n' drop some files here, or click to select files</p>
        <em>(Only files with name less than 20 characters will be accepted)</em>
      </div>
      <aside>
        <h4>Accepted files</h4>
        <ul>{acceptedFileItems}</ul>
        <h4>Rejected files</h4>
        <ul>{fileRejectionItems}</ul>
      </aside>
    </section>
  );
}

<CustomValidation />
```