diff --git a/yt_dlp/postprocessor/embedthumbnail.py b/yt_dlp/postprocessor/embedthumbnail.py
index f3f46bbeabe..88a767132ac 100644
--- a/yt_dlp/postprocessor/embedthumbnail.py
+++ b/yt_dlp/postprocessor/embedthumbnail.py
@@ -107,7 +107,7 @@ def run(self, info):
                 options.extend(['-map', '-0:%d' % old_stream])
                 new_stream -= 1
             options.extend([
-                '-attach', f'file:{thumbnail_filename}',
+                '-attach', self._ffmpeg_filename_argument(thumbnail_filename),
                 '-metadata:s:%d' % new_stream, 'mimetype=%s' % mimetype,
                 '-metadata:s:%d' % new_stream, 'filename=cover.%s' % thumbnail_ext])
 
diff --git a/yt_dlp/postprocessor/ffmpeg.py b/yt_dlp/postprocessor/ffmpeg.py
index 508300da2f3..63fc9ace65b 100644
--- a/yt_dlp/postprocessor/ffmpeg.py
+++ b/yt_dlp/postprocessor/ffmpeg.py
@@ -809,7 +809,7 @@ def _get_infojson_opts(self, info, infofn):
             new_stream -= 1
 
         yield (
-            '-attach', f'file:{infofn}',
+            '-attach', self._ffmpeg_filename_argument(infofn),
             f'-metadata:s:{new_stream}', 'mimetype=application/json',
             f'-metadata:s:{new_stream}', 'filename=info.json',
         )