namespace WageCategorylistSetup;

use MagentoFrameworkModuleSetupMigration;
use MagentoFrameworkSetupInstallDataInterface;
use MagentoFrameworkSetupModuleContextInterface;
use MagentoFrameworkSetupModuleDataSetupInterface;
use MagentoCatalogSetupCategorySetupFactory;

class InstallData implements InstallDataInterface
{
    public function __construct(CategorySetupFactory $categorySetupFactory)
    {
        $this->categorySetupFactory = $categorySetupFactory;
    }
    public function install(ModuleDataSetupInterface $setup, ModuleContextInterface $context)
    {
        $installer = $setup;
        $installer->startSetup();

        $categorySetup = $this->categorySetupFactory->create(['setup' => $setup]);
        $entityTypeId = $categorySetup->getEntityTypeId(MagentoCatalogModelCategory::ENTITY);
        $attributeSetId = $categorySetup->getDefaultAttributeSetId($entityTypeId);
        $categorySetup->removeAttribute(
            MagentoCatalogModelCategory::ENTITY, 'banner_image' );
        $categorySetup->addAttribute(
            MagentoCatalogModelCategory::ENTITY, 'banner_image', [
                'type' => 'varchar',
                'label' => 'Featured Image',
                'input' => 'image',
                'backend' => 'MagentoCatalogModelCategoryAttributeBackendImage',
                'required' => false,
                'sort_order' => 5,
                'global' => MagentoEavModelEntityAttributeScopedAttributeInterface::SCOPE_STORE,
                'group' => 'General Information',
            ]
        );
        $installer->endSetup();
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<form xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Ui:etc/ui_configuration.xsd">
    <fieldset name="content">
        <field name="thumbnail">
            <argument name="data" xsi:type="array">
                <item name="config" xsi:type="array">
                    <item name="dataType" xsi:type="string">string</item>
                    <item name="source" xsi:type="string">category</item>
                    <item name="label" xsi:type="string" translate="true">Thumbnail Image</item>
                    <item name="visible" xsi:type="boolean">true</item>
                    <item name="formElement" xsi:type="string">fileUploader</item>
                    <item name="elementTmpl" xsi:type="string">ui/form/element/uploader/uploader</item>
                    <item name="previewTmpl" xsi:type="string">Magento_Catalog/image-preview</item>
                    <item name="required" xsi:type="boolean">false</item>
                    <item name="sortOrder" xsi:type="number">30</item>
                    <item name="uploaderConfig" xsi:type="array">
                        <item name="url" xsi:type="url" path="categorylist/category_thumbnailimage/upload"/>
                    </item>
                </item>
            </argument>
        </field>
    </fieldset>
</form>

namespace WageCategorylistControllerAdminhtmlCategoryThumbnailimage;

use MagentoFrameworkControllerResultFactory;

/**
 * Class Upload
 */
class Upload extends MagentoBackendAppAction
{
    protected $baseTmpPath;
    protected $imageUploader;
    public function __construct(
        MagentoBackendAppActionContext $context,
        MagentoCatalogModelImageUploader $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);
   }
}

<?xml version="1.0"?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="WageCategorylistControllerAdminhtmlCategoryThumbnailimageUpload">
        <arguments>
            <argument name="imageUploader" xsi:type="object">MagentoCatalogCategoryImageUpload</argument>
        </arguments>
    </type>
    <virtualType name="MagentoCatalogCategoryImageUpload" type="MagentoCatalogModelImageUploader">
        <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>
</config>