Magento 2 Image upload on CMS page

I had the same problem, the fileuploader uses a specific array to preview the image and you have to create it manually.

You have to extend the source defined in:

<item name="source" xsi:type="string">page</item>

And extend the DataProvider in di.xml:

<preference for="Magento\Cms\Model\Page\DataProvider" type="Dexanet\Cms\Model\Page\DataProvider"/>

You have to extend the getData() method like:

public function getData() {
  $data = parent::getData();

  foreach ($data as $pageId => $page) {
    if (isset($page['header_image'])) {
      unset($data[$pageId]['header_image']);
      $fileName = $page['header_image'];

      $serverPath = $this->fileSystem->getDirectoryRead(DirectoryList::MEDIA)
          ->getAbsolutePath() . $this->imageUploader->getBasePath() . "/" . $fileName;
      $fullPath = $this->storeManager->getStore()->getBaseUrl(
          \Magento\Framework\UrlInterface::URL_TYPE_MEDIA
        ) . $this->imageUploader->getBasePath() . "/" . $fileName;

      if (file_exists($serverPath)) {
        $stat = stat($serverPath);
        $mime = mime_content_type($serverPath);

        $data[$pageId]["header_image"][0]['name'] = $fileName;
        $data[$pageId]["header_image"][0]['url'] = $fullPath;
        $data[$pageId]['header_image'][0]['size'] = isset($stat) ? $stat['size'] : 0;
        $data[$pageId]['header_image'][0]['type'] = $mime;
      }
    }
  }

  return $data;
}