diff --git a/src/Model/Photo.php b/src/Model/Photo.php
index fe416c360f..61fc2df4ea 100644
--- a/src/Model/Photo.php
+++ b/src/Model/Photo.php
@@ -245,13 +245,17 @@ class Photo
 	 * Construct a photo array for a system resource image
 	 *
 	 * @param string $filename Image file name relative to code root
-	 * @param string $mimetype Image mime type. Defaults to "image/jpeg"
+	 * @param string $mimetype Image mime type. Is guessed by file name when empty.
 	 *
 	 * @return array
 	 * @throws \Exception
 	 */
-	public static function createPhotoForSystemResource($filename, $mimetype = "image/jpeg")
+	public static function createPhotoForSystemResource($filename, $mimetype = '')
 	{
+		if (empty($mimetype)) {
+			$mimetype = Images::guessTypeByExtension($filename);
+		}
+
 		$fields = self::getFields();
 		$values = array_fill(0, count($fields), "");
 
@@ -269,13 +273,17 @@ class Photo
 	 *
 	 * @param string $url      Image URL
 	 * @param int    $uid      User ID of the requesting person
-	 * @param string $mimetype Image mime type. Defaults to "image/jpeg"
+	 * @param string $mimetype Image mime type. Is guessed by file name when empty.
 	 *
 	 * @return array
 	 * @throws \Exception
 	 */
-	public static function createPhotoForExternalResource($url, $uid = 0, $mimetype = "image/jpeg")
+	public static function createPhotoForExternalResource($url, $uid = 0, $mimetype = '')
 	{
+		if (empty($mimetype)) {
+			$mimetype = Images::guessTypeByExtension($url);
+		}
+
 		$fields = self::getFields();
 		$values = array_fill(0, count($fields), "");
 
diff --git a/src/Module/Photo.php b/src/Module/Photo.php
index e5042fad4e..3b4b7cc569 100644
--- a/src/Module/Photo.php
+++ b/src/Module/Photo.php
@@ -29,6 +29,7 @@ use Friendica\Model\Contact;
 use Friendica\Model\Photo as MPhoto;
 use Friendica\Model\Post;
 use Friendica\Model\Storage\ExternalResource;
+use Friendica\Model\Storage\SystemResource;
 use Friendica\Util\Proxy;
 use Friendica\Object\Image;
 use Friendica\Util\Images;
@@ -102,8 +103,8 @@ class Photo extends BaseModule
 		$stamp = microtime(true);
 		$imgdata = MPhoto::getImageDataForPhoto($photo);
 
-		// The mimetype for an external resource can only be known after it had been fetched
-		if ($photo['backend-class'] == ExternalResource::NAME) {
+		// The mimetype for an external or system resource can only be known reliably after it had been fetched
+		if (in_array($photo['backend-class'], [ExternalResource::NAME, SystemResource::NAME])) {
 			$mimetype = Images::getMimeTypeByData($imgdata);
 			if (!empty($mimetype)) {
 				$photo['type'] = $mimetype;