[bellazon] improve 'filename' & 'extension' (#8544)

https://github.com/mikf/gallery-dl/issues/8544#issuecomment-3522617104

- use 'extension' from file URL (except attachments)
- strip everything after the first '.' from non-URL filenames

test/results/bellazon.py

@@ -237,6 +237,17 @@ __tests__ = (
     "id"          : "14418097",
 },
 
+{
+    "#url"     : "https://www.bellazon.com/main/topic/4322-candids/page/1066/#comment-3956772",
+    "#comment" : "weird/wrong 'filename' & 'extension' (#8544)",
+    "#class"   : bellazon.BellazonPostExtractor,
+    "#count"   : 16,
+
+    "extension"   : "jpg",
+    "filename"    : r"re:^[^.]+$",
+    "id"          : r"re:^\d+$",
+},
+
 {
     "#url"     : "https://www.bellazon.com/main/topic/57872-millie-brady/",
     "#class"   : bellazon.BellazonThreadExtractor,