diff --git a/core/imageboard/image.php b/core/imageboard/image.php
index b65dc8ef..5d0e33c5 100644
--- a/core/imageboard/image.php
+++ b/core/imageboard/image.php
@@ -598,7 +598,7 @@ class Image
     /**
      * Get the image's mime type.
      */
-    public function get_mime(): string
+    public function get_mime(): ?string
     {
         if ($this->mime===MimeType::WEBP&&$this->lossless) {
             return MimeType::WEBP_LOSSLESS;
diff --git a/ext/mime/main.php b/ext/mime/main.php
index bcbccdde..c98adde5 100644
--- a/ext/mime/main.php
+++ b/ext/mime/main.php
@@ -45,7 +45,7 @@ class MimeSystem extends Extension
                 $normalized_extension = FileExtension::get_for_mime($mime);
 
                 $database->execute(
-                    "UPDATE images SET mime = :mime, ext = :new_ext WHERE ext = :ext AND (mime != :mime OR ext != :new_ext)",
+                    "UPDATE images SET mime = :mime, ext = :new_ext WHERE ext = :ext AND (mime IS NULL OR mime != :mime OR ext != :new_ext)",
                     ["mime" => $mime, "new_ext" => $normalized_extension, "ext" => $ext]
                 );
             }