To create category attribute thumbnail image, following code snippet can be used to serve the purpose.

You need to create a controller file as following:

<?php

namespace Magemeta\Categorylist\Controller\Adminhtml\Category\Thumbnailimage;

use Magento\Framework\Controller\ResultFactory;

/**
 * Class Upload
 */
class Upload extends \Magento\Backend\App\Action
{
    protected $baseTmpPath;
      /**
     * Image uploader
     *
     * @var \Magento\Catalog\Model\ImageUploader
     */
    protected $imageUploader;

    /**
     * Upload constructor.
     *
     * @param \Magento\Backend\App\Action\Context $context
     * @param \Magento\Catalog\Model\ImageUploader $imageUploader
     */
    public function __construct(
        \Magento\Backend\App\Action\Context $context,
        \Magento\Catalog\Model\ImageUploader $imageUploader
    ) {
        $this->imageUploader = $imageUploader;
        parent::__construct($context);

    }
   public function execute() {
        try {
            $result = $this->imageUploader->saveFileToTmpDir('thumbnail');
            $result['cookie'] = [
                'name' => $this->_getSession()->getName(),
                'value' => $this->_getSession()->getSessionId(),
                'lifetime' => $this->_getSession()->getCookieLifetime(),
                'path' => $this->_getSession()->getCookiePath(),
                'domain' => $this->_getSession()->getCookieDomain(),
            ];
        } catch (\Exception $e) {
            $result = ['error' => $e->getMessage(), 'errorcode' => $e->getCode()];
        }
        return $this->resultFactory->create(ResultFactory::TYPE_JSON)->setData($result);
   }
}

You need to add this in the di.xml of your module:

<type name="Magemeta\Categorylist\Controller\Adminhtml\Category\Thumbnailimage\Upload">
    <arguments>
        <argument name="imageUploader" xsi:type="object">Magento\Catalog\CategoryImageUpload</argument>
    </arguments>
</type>

This is needed because the dependency of your controller called imageUploader is an instance of \Magento\Catalog\Model\ImageUploader. This is just a general class for upload that does not have the base path and base tmp path set. So it does not know where to upload the files.
But in the etc/di.xml file of the catalog module you have this virtual type defined:

<virtualType name="Magento\Catalog\CategoryImageUpload" type="Magento\Catalog\Model\ImageUploader">
    <arguments>
        <argument name="baseTmpPath" xsi:type="string">catalog/tmp/category</argument>
        <argument name="basePath" xsi:type="string">catalog/category</argument>
        <argument name="allowedExtensions" xsi:type="array">
            <item name="jpg" xsi:type="string">jpg</item>
            <item name="jpeg" xsi:type="string">jpeg</item>
            <item name="gif" xsi:type="string">gif</item>
            <item name="png" xsi:type="string">png</item>
        </argument>
    </arguments>
</virtualType>