diff --git a/runtime/lib/precache-webdav-previews.php b/runtime/lib/precache-webdav-previews.php
index 078207d..aed8fbf 100644
--- a/runtime/lib/precache-webdav-previews.php
+++ b/runtime/lib/precache-webdav-previews.php
@@ -6,6 +6,10 @@ if (PHP_SAPI !== 'cli')
   exit(1);
 }
 
+const BRATONIEN_WEBDAV_PREVIEW_VERSION = 2;
+const BRATONIEN_WEBDAV_PREVIEW_MAX_EDGE = 4096;
+const BRATONIEN_WEBDAV_PREVIEW_JPEG_QUALITY = 88;
+
 function fail_preview($message)
 {
   throw new RuntimeException($message);
@@ -28,72 +32,161 @@ function fetch_remote_blob($url, $user, $password)
     CURLOPT_HTTPAUTH => CURLAUTH_BASIC,
     CURLOPT_USERPWD => $user.':'.$password,
     CURLOPT_FAILONERROR => false,
-    CURLOPT_USERAGENT => 'Bratonien-Tools-WebDAV-Precache/0.9.5.19',
+    CURLOPT_USERAGENT => 'Bratonien-Tools-WebDAV-Precache/0.9.6.0',
   ));
   $body = curl_exec($ch);
   $errno = curl_errno($ch);
   $error = curl_error($ch);
   $http = (int)curl_getinfo($ch, CURLINFO_HTTP_CODE);
   curl_close($ch);
+
   if ($body === false || $errno !== 0) fail_preview('WebDAV-Bild konnte nicht geladen werden: '.$error);
   if ($http < 200 || $http >= 300) fail_preview('WebDAV-Bild antwortete mit HTTP '.$http.'.');
+  if ($body === '') fail_preview('WebDAV-Bild ist leer.');
   return (string)$body;
 }
 
-function write_preview($blob, $target)
+function preview_extension_for_entry(array $entry)
+{
+  $content_type = strtolower(trim((string)($entry['content_type'] ?? '')));
+  $path = strtolower((string)($entry['webdav_path'] ?? ''));
+  if ($content_type === 'image/png' || preg_match('/\.png$/', $path))
+  {
+    return 'png';
+  }
+  return 'jpg';
+}
+
+function preview_target_for_entry($cache_dir, $key, array $entry)
+{
+  return $cache_dir.'/'.$key.'.'.preview_extension_for_entry($entry);
+}
+
+function remove_other_preview_format($cache_dir, $key, $keep_target)
+{
+  foreach (array('jpg', 'png', 'webp') as $ext)
+  {
+    $candidate = $cache_dir.'/'.$key.'.'.$ext;
+    if ($candidate !== $keep_target && is_file($candidate)) @unlink($candidate);
+  }
+}
+
+function write_preview_imagick($blob, $target, $extension)
+{
+  $image = new Imagick();
+  try
+  {
+    $image->readImageBlob($blob);
+    if (method_exists($image, 'autoOrientImage')) @$image->autoOrientImage();
+    $image->setIteratorIndex(0);
+    $image->thumbnailImage(BRATONIEN_WEBDAV_PREVIEW_MAX_EDGE, BRATONIEN_WEBDAV_PREVIEW_MAX_EDGE, true, true);
+    $image->stripImage();
+
+    if ($extension === 'png')
+    {
+      $image->setImageFormat('png');
+    }
+    else
+    {
+      $image->setImageBackgroundColor('white');
+      if (method_exists($image, 'mergeImageLayers'))
+      {
+        $image = $image->mergeImageLayers(Imagick::LAYERMETHOD_FLATTEN);
+      }
+      $image->setImageFormat('jpeg');
+      $image->setImageCompression(Imagick::COMPRESSION_JPEG);
+      $image->setImageCompressionQuality(BRATONIEN_WEBDAV_PREVIEW_JPEG_QUALITY);
+      $image->setInterlaceScheme(Imagick::INTERLACE_PLANE);
+    }
+
+    if (!$image->writeImage($target)) fail_preview('Imagick konnte das Preview nicht schreiben.');
+  }
+  finally
+  {
+    $image->clear();
+    $image->destroy();
+  }
+}
+
+function write_preview_gd($blob, $target, $extension)
+{
+  $source = @imagecreatefromstring($blob);
+  if (!$source) fail_preview('GD konnte das Bild nicht dekodieren.');
+
+  try
+  {
+    $width = imagesx($source);
+    $height = imagesy($source);
+    if ($width < 1 || $height < 1) fail_preview('Bildabmessungen sind ungültig.');
+
+    $scale = min(1, BRATONIEN_WEBDAV_PREVIEW_MAX_EDGE / $width, BRATONIEN_WEBDAV_PREVIEW_MAX_EDGE / $height);
+    $target_width = max(1, (int)round($width * $scale));
+    $target_height = max(1, (int)round($height * $scale));
+    $preview = imagecreatetruecolor($target_width, $target_height);
+    if (!$preview) fail_preview('GD konnte die Preview-Arbeitsfläche nicht anlegen.');
+
+    try
+    {
+      if ($extension === 'png')
+      {
+        imagealphablending($preview, false);
+        imagesavealpha($preview, true);
+        $transparent = imagecolorallocatealpha($preview, 0, 0, 0, 127);
+        imagefilledrectangle($preview, 0, 0, $target_width, $target_height, $transparent);
+      }
+      else
+      {
+        $white = imagecolorallocate($preview, 255, 255, 255);
+        imagefilledrectangle($preview, 0, 0, $target_width, $target_height, $white);
+      }
+
+      if (!imagecopyresampled($preview, $source, 0, 0, 0, 0, $target_width, $target_height, $width, $height))
+      {
+        fail_preview('GD konnte das Preview nicht skalieren.');
+      }
+
+      $written = $extension === 'png'
+        ? imagepng($preview, $target, 6)
+        : imagejpeg($preview, $target, BRATONIEN_WEBDAV_PREVIEW_JPEG_QUALITY);
+      if (!$written) fail_preview('GD konnte das Preview nicht schreiben.');
+    }
+    finally
+    {
+      imagedestroy($preview);
+    }
+  }
+  finally
+  {
+    imagedestroy($source);
+  }
+}
+
+function write_preview($blob, $target, $extension)
 {
   $dir = dirname($target);
   if (!is_dir($dir) && !mkdir($dir, 0755, true) && !is_dir($dir)) fail_preview('Preview-Verzeichnis konnte nicht angelegt werden.');
 
   if (class_exists('Imagick'))
   {
-    $image = new Imagick();
-    $image->readImageBlob($blob);
-    if (method_exists($image, 'autoOrientImage')) @$image->autoOrientImage();
-    $image->setIteratorIndex(0);
-    $image->thumbnailImage(1600, 1600, true, true);
-    $image->setImageFormat('webp');
-    $image->setImageCompressionQuality(85);
-    if (!$image->writeImage($target)) fail_preview('Imagick konnte das Preview nicht schreiben.');
-    $image->clear();
-    $image->destroy();
-    @chmod($target, 0644);
-    return;
+    write_preview_imagick($blob, $target, $extension);
   }
-
-  if (function_exists('imagecreatefromstring') && function_exists('imagewebp'))
+  elseif (function_exists('imagecreatefromstring') && function_exists('imagejpeg') && function_exists('imagepng'))
   {
-    $source = @imagecreatefromstring($blob);
-    if (!$source) fail_preview('GD konnte das Bild nicht dekodieren.');
-    $width = imagesx($source);
-    $height = imagesy($source);
-    if ($width < 1 || $height < 1)
-    {
-      imagedestroy($source);
-      fail_preview('Bildabmessungen sind ungültig.');
-    }
-    $scale = min(1, 1600 / $width, 1600 / $height);
-    $target_width = max(1, (int)round($width * $scale));
-    $target_height = max(1, (int)round($height * $scale));
-    $preview = imagecreatetruecolor($target_width, $target_height);
-    imagealphablending($preview, false);
-    imagesavealpha($preview, true);
-    $transparent = imagecolorallocatealpha($preview, 0, 0, 0, 127);
-    imagefilledrectangle($preview, 0, 0, $target_width, $target_height, $transparent);
-    imagecopyresampled($preview, $source, 0, 0, 0, 0, $target_width, $target_height, $width, $height);
-    if (!imagewebp($preview, $target, 85))
-    {
-      imagedestroy($preview);
-      imagedestroy($source);
-      fail_preview('GD konnte das Preview nicht schreiben.');
-    }
-    imagedestroy($preview);
-    imagedestroy($source);
-    @chmod($target, 0644);
-    return;
+    write_preview_gd($blob, $target, $extension);
+  }
+  else
+  {
+    fail_preview('Weder Imagick noch GD ist für die Preview-Erzeugung verfügbar.');
   }
 
-  fail_preview('Weder Imagick noch GD/WebP ist für die Preview-Erzeugung verfügbar.');
+  clearstatcache(true, $target);
+  $size = @getimagesize($target);
+  if (!is_array($size) || empty($size[0]) || empty($size[1]) || !is_file($target) || filesize($target) < 64)
+  {
+    @unlink($target);
+    fail_preview('Das erzeugte Preview ist ungültig.');
+  }
+  @chmod($target, 0644);
 }
 
 try
@@ -117,6 +210,7 @@ try
 
   if (!is_dir($cache_dir) && !mkdir($cache_dir, 0755, true) && !is_dir($cache_dir)) fail_preview('Preview-Cache konnte nicht angelegt werden.');
   @chmod($cache_dir, 0755);
+
   $state_file = $cache_dir.'/state.json';
   $old_state = array();
   if (is_readable($state_file))
@@ -135,12 +229,26 @@ try
     if (!is_array($entry) || (string)($entry['kind'] ?? '') !== 'file') continue;
     $webdav_path = trim((string)($entry['webdav_path'] ?? ''), '/');
     if ($webdav_path === '') continue;
+
     $etag = (string)($entry['etag'] ?? '');
     $key = sha1($webdav_path);
-    $target = $cache_dir.'/'.$key.'.webp';
-    $new_state[$key] = array('path'=>$webdav_path, 'etag'=>$etag);
+    $target = preview_target_for_entry($cache_dir, $key, $entry);
+    $extension = pathinfo($target, PATHINFO_EXTENSION);
+    $new_state[$key] = array(
+      'path' => $webdav_path,
+      'etag' => $etag,
+      'format' => $extension,
+      'version' => BRATONIEN_WEBDAV_PREVIEW_VERSION,
+    );
 
-    if (is_file($target) && isset($old_state[$key]) && (string)($old_state[$key]['etag'] ?? '') === $etag)
+    $old = $old_state[$key] ?? null;
+    if (
+      is_file($target)
+      && is_array($old)
+      && (string)($old['etag'] ?? '') === $etag
+      && (string)($old['format'] ?? '') === $extension
+      && (int)($old['version'] ?? 0) === BRATONIEN_WEBDAV_PREVIEW_VERSION
+    )
     {
       $cached++;
       continue;
@@ -150,7 +258,8 @@ try
     {
       $url = $base_url.'/remote.php/dav/files/'.rawurlencode($user).'/'.quote_webdav_path($webdav_path);
       $blob = fetch_remote_blob($url, $user, $password);
-      write_preview($blob, $target);
+      write_preview($blob, $target, $extension);
+      remove_other_preview_format($cache_dir, $key, $target);
       $generated++;
     }
     catch (Throwable $e)
@@ -160,10 +269,12 @@ try
     }
   }
 
-  foreach (glob($cache_dir.'/*.webp') ?: array() as $file)
+  foreach (glob($cache_dir.'/*') ?: array() as $file)
   {
-    $key = basename($file, '.webp');
-    if (!isset($new_state[$key])) @unlink($file);
+    if (!is_file($file) || basename($file) === 'state.json') continue;
+    $name = basename($file);
+    if (!preg_match('/^([a-f0-9]{40})\.(jpg|png|webp)$/', $name, $m)) continue;
+    if (!isset($new_state[$m[1]])) @unlink($file);
   }
 
   file_put_contents($state_file, json_encode($new_state, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT)."\n", LOCK_EX);