diff --git a/inc/functions/metadata.php b/inc/functions/metadata.php
index 9abfecb7..e0a267c2 100644
--- a/inc/functions/metadata.php
+++ b/inc/functions/metadata.php
@@ -14,6 +14,14 @@ const SUPPORTED_IMAGE_MIME_TYPES = [
 	'image/avif'
 ];
 
+/**
+ * Non-image file types for which only thumbnail generation is supported.
+ */
+const SUPPORTED_THUMB_MIME_TYPES = [
+	'text/plain',
+	'application/pdf'
+];
+
 /**
  * @param string $mime Lowercase valid mime type.
  * @return ?string Vichan's preferred extension for the given mime type, if any.
@@ -25,7 +33,9 @@ function mime_to_ext(string $mime): ?string {
 		'image/gif' => 'gif',
 		'image/webp' => 'webp',
 		'image/bmp' => 'bmp',
-		'image/avif' => 'avif'
+		'image/avif' => 'avif',
+		'text/plain' => 'txt',
+		'application/pdf' => 'pdf'
 	];
 	return $mime_to_ext[$mime] ?? null;
 }
@@ -43,6 +53,8 @@ function ext_to_mime(string $ext): ?string {
 		'webp' => 'image/webp',
 		'bmp' => 'image/bmp',
 		'avif' => 'image/avif',
+		'txt' => 'text/plain',
+		'pdf' => 'application/pdf'
 	];
 	return $ext_to_mime[$ext] ?? null;
 }