diff --git a/.gitignore b/.gitignore
index 8fcd0de641..40bb34d2aa 100644
--- a/.gitignore
+++ b/.gitignore
@@ -105,6 +105,8 @@ README.txt
 *.zsh
 *.spec
 test/testdata/sigs/player-*.js
+test/testdata/thumbnails/empty.webp
+test/testdata/thumbnails/foo\ %d\ bar/foo_%d.*
 
 # Binary
 /youtube-dl
diff --git a/Makefile b/Makefile
index 6c72ead1ef..273cb3cc0b 100644
--- a/Makefile
+++ b/Makefile
@@ -18,10 +18,11 @@ pypi-files: AUTHORS Changelog.md LICENSE README.md README.txt supportedsites \
         tar pypi-files lazy-extractors install uninstall
 
 clean-test:
-	rm -rf test/testdata/sigs/player-*.js tmp/ *.annotations.xml *.aria2 *.description *.dump *.frag \
+	rm -rf tmp/ *.annotations.xml *.aria2 *.description *.dump *.frag \
 	*.frag.aria2 *.frag.urls *.info.json *.live_chat.json *.meta *.part* *.tmp *.temp *.unknown_video *.ytdl \
 	*.3gp *.ape *.ass *.avi *.desktop *.f4v *.flac *.flv *.gif *.jpeg *.jpg *.lrc *.m4a *.m4v *.mhtml *.mkv *.mov *.mp3 *.mp4 \
-	*.mpg *.mpga *.oga *.ogg *.opus *.png *.sbv *.srt *.ssa *.swf *.tt *.ttml *.url *.vtt *.wav *.webloc *.webm *.webp
+	*.mpg *.mpga *.oga *.ogg *.opus *.png *.sbv *.srt *.ssa *.swf *.tt *.ttml *.url *.vtt *.wav *.webloc *.webm *.webp \
+	test/testdata/sigs/player-*.js test/testdata/thumbnails/empty.webp "test/testdata/thumbnails/foo %d bar/foo_%d."*
 clean-dist:
 	rm -rf yt-dlp.1.temp.md yt-dlp.1 README.txt MANIFEST build/ dist/ .coverage cover/ yt-dlp.tar.gz completions/ \
 	yt_dlp/extractor/lazy_extractors.py *.spec CONTRIBUTING.md.tmp yt-dlp yt-dlp.exe yt_dlp.egg-info/ AUTHORS
diff --git a/test/test_postprocessors.py b/test/test_postprocessors.py
index 603f85c654..ecc73e39eb 100644
--- a/test/test_postprocessors.py
+++ b/test/test_postprocessors.py
@@ -8,6 +8,8 @@ import unittest
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 
+import subprocess
+
 from yt_dlp import YoutubeDL
 from yt_dlp.utils import shell_quote
 from yt_dlp.postprocessor import (
@@ -47,7 +49,18 @@ class TestConvertThumbnail(unittest.TestCase):
             print('Skipping: ffmpeg not found')
             return
 
-        file = 'test/testdata/thumbnails/foo %d bar/foo_%d.{}'
+        test_data_dir = 'test/testdata/thumbnails'
+        generated_file = f'{test_data_dir}/empty.webp'
+
+        subprocess.check_call([
+            pp.executable, '-y', '-f', 'lavfi', '-i', 'color=c=black:s=320x320',
+            '-c:v', 'libwebp', '-pix_fmt', 'yuv420p', '-vframes', '1', generated_file,
+        ], stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
+
+        file = test_data_dir + '/foo %d bar/foo_%d.{}'
+        initial_file = file.format('webp')
+        os.replace(generated_file, initial_file)
+
         tests = (('webp', 'png'), ('png', 'jpg'))
 
         for inp, out in tests:
@@ -55,11 +68,13 @@ class TestConvertThumbnail(unittest.TestCase):
             if os.path.exists(out_file):
                 os.remove(out_file)
             pp.convert_thumbnail(file.format(inp), out)
-            assert os.path.exists(out_file)
+            self.assertTrue(os.path.exists(out_file))
 
         for _, out in tests:
             os.remove(file.format(out))
 
+        os.remove(initial_file)
+
 
 class TestExec(unittest.TestCase):
     def test_parse_cmd(self):
@@ -610,3 +625,7 @@ outpoint 10.000000
         self.assertEqual(
             r"'special '\'' characters '\'' galore'\'\'\'",
             self._pp._quote_for_ffmpeg("special ' characters ' galore'''"))
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/test/testdata/thumbnails/foo %d bar/foo_%d.webp b/test/testdata/thumbnails/foo %d bar/foo_%d.webp
deleted file mode 100644
index d64d0839f0..0000000000
Binary files a/test/testdata/thumbnails/foo %d bar/foo_%d.webp and /dev/null differ
diff --git a/test/testdata/thumbnails/foo %d bar/placeholder b/test/testdata/thumbnails/foo %d bar/placeholder
new file mode 100644
index 0000000000..e69de29bb2