improve isfile validation check (#3101)

tests/helpers/test_config_validation.py

@@ -1,4 +1,6 @@
 from datetime import timedelta
+import os
+import tempfile
 
 import pytest
 import voluptuous as vol
@@ -59,6 +61,24 @@ def test_port():
         schema(value)
 
 
+def test_isfile():
+    """Validate that the value is an existing file."""
+    schema = vol.Schema(cv.isfile)
+
+    with tempfile.NamedTemporaryFile() as fp:
+        pass
+
+    for value in ('invalid', None, -1, 0, 80000, fp.name):
+        with pytest.raises(vol.Invalid):
+            schema(value)
+
+    with tempfile.TemporaryDirectory() as tmp_path:
+        tmp_file = os.path.join(tmp_path, "test.txt")
+        with open(tmp_file, "w") as tmp_handl:
+            tmp_handl.write("test file")
+        schema(tmp_file)
+
+
 def test_url():
     """Test URL."""
     schema = vol.Schema(cv.url)