Add full stop to reserved character list

CHANGELOG.md

@@ -17,7 +17,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 - `MouseScrollUp` and `MouseScrollDown` now inherit from `MouseEvent` and have attached modifier keys. https://github.com/Textualize/textual/pull/1458
 - Fail-fast and print pretty tracebacks for Widget compose errors https://github.com/Textualize/textual/pull/1505
-- The default filename for screenshots uses ISO8601 datetime format, with colons and spaces replaced with underscores https://github.com/Textualize/textual/pull/1518
+- The default filename for screenshots uses a datetime format similar to ISO8601, but with reserved characters replaced by underscores https://github.com/Textualize/textual/pull/1518
 
 ### Fixed
 

src/textual/app.py

@@ -663,9 +663,10 @@ class App(Generic[ReturnType], DOMNode):
                 dt = datetime.now().isoformat()
             else:
                 dt = datetime.now().strftime(time_format)
-            svg_filename = f"{self.title.lower()} {dt}.svg"
-            for reserved in ' <>:"/\\|?*':
-                svg_filename = svg_filename.replace(reserved, "_")
+            svg_filename_stem = f"{self.title.lower()} {dt}"
+            for reserved in ' <>:"/\\|?*.':
+                svg_filename_stem = svg_filename_stem.replace(reserved, "_")
+                svg_filename = svg_filename_stem + ".svg"
         else:
             svg_filename = filename
         svg_path = os.path.expanduser(os.path.join(path, svg_filename))