Drupal 7 MIME type missed validation function

missed_drupal_mime_validation.php

<?php
/**
 * Checks that the file has allowed MIME type.
 *
 * @param $file
 *   A Drupal file object.
 * @param $mimetypes_string
 *   A string with a space separated list of allowed MIME types.
 *
 * @return
 *   An array. If the file MIME type is not allowed, it will contain an error
 *   message.
 *
 * @see hook_file_validate()
 */
function mymodule_mimetype_validate(stdClass $file, $mimetypes_string) {
  $errors = array(); //At begin our errors array is empty. If it will remain empty at the end of the function -> we conclude here is no errors with MIME type.
  $allowed_mime_types = explode(' ', $mimetypes_string);

  if (!in_array(mime_content_type($file->uri), $allowed_mime_types)) {
    $errors[] = t('Only files of following MIME types are allowed: %files-allowed.', array('%files-allowed' => $mimetypes_string));
  }

  return $errors;
}

//Usage example. Such function can be attached to form submit button as custom validate function:

function mymodule_validate_csv($form, &$form_state) {
  //At first - check $_FILES array and if there is no file -> set_form_error().
  //Other errors will be handled in file_save_upload(), see: https://api.drupal.org/api/drupal/includes%21file.inc/function/file_save_upload/7.x
  if ($_FILES['files']['error']['file'] === UPLOAD_ERR_NO_FILE) {
    form_set_error('file', t('File field can\'t be empty.'));
  }

  //This is array of validation functions and their arguments, this array will be passed as second argument to file_save_upload().
  //"file_validate_extensions" is standard system validator, "mymodule_mimetype_validate" is our custom one.
  $validators_array = array(
    'file_validate_extensions' => array('csv txt'),
    'mymodule_mimetype_validate' => array('text/plain'),
  );

  $file = file_save_upload('file', $validators_array, 'public://');

  if ($file) {
    //Save the file for use in the submit handler.
    $form_state['storage']['file'] = $file;
    drupal_set_message(t('File successfully uploaded.'));
  } else {
    form_set_error('file', t('File was not uploaded.'));
  }
}