diff --git a/tinymce.pages.inc b/tinymce.pages.inc
index e7d9eb2..e8a0dc0 100644
--- a/tinymce.pages.inc
+++ b/tinymce.pages.inc
@@ -28,8 +28,10 @@ function tinymce_image_upload($format) {
   }
 
   $destination = $upload_settings['scheme'] . '://' .  $upload_settings['directory'];
+  $extensions = image_get_supported_extensions();
   $validators = array(
-    'file_validate_is_image' => array(),
+    'file_validate_extensions' => array(implode(' ', $extensions)),
+    'tinymce_validate_is_image' => array(),
   );
   if ($upload_settings['max_size']) {
     $validators['file_validate_size'] = array(parse_size($upload_settings['max_size']));
@@ -52,8 +54,8 @@ function tinymce_image_upload($format) {
       'uploaded' => 1,
       'location' => $url,
       'fileId' => $file->fid,
-      'width' => $image_info['width'],
-      'height' => $image_info['height'],
+      'width' => !empty($image_info['width']) ? $image_info['width'] : '',
+      'height' => !empty($image_info['height']) ? $image_info['height'] : '',
     );
   }
   else {
@@ -73,3 +75,28 @@ function tinymce_image_upload($format) {
 
   return $response;
 }
+
+/**
+ * Custom validation callback for image upload.
+ *
+ * Currently file_validate_is_image() fails for newly uploaded files, as the uri
+ * is something like "/tmp/phpAbCde" - without file extension.
+ * @see https://github.com/backdrop/backdrop-issues/issues/6497
+ */
+function tinymce_validate_is_image(File $file) {
+  $errors = array();
+
+  // SVG have their own validator on upload (file_validate_svg).
+  // If SVG aren't supported, yet, TinyMCE would refuse to upload in the first
+  // place and even if upload was attempted, the server-side extension based
+  // validation would fail.
+  if ($file->filemime == 'image/svg+xml') {
+    return $errors;
+  }
+  // Validate that the image toolkit can handle this raster image.
+  if (!$info = image_get_info($file->uri)) {
+    $errors[] = t('The image appears to be broken.');
+  }
+
+  return $errors;
+}