FileChooser: Avoid setting default extension to filename

Fixes a bug on Windows when opening a FileChooser with the following
arguments.

    FileChooser ("",
                 "C:\path\to\file", // filename 'file' doesn't contain '.'
                 "",                // all extensions are allowed
                 true);

The expected behaviour is that the initially-displayed filename is
"file".

The actual behaviour was that the initially-displayed filename was
"file.file".

modules/juce_gui_basics/native/juce_win32_FileChooser.cpp

@@ -581,19 +581,20 @@ private:
 
     String getDefaultFileExtension (const String& filename) const
     {
-        auto extension = filename.fromLastOccurrenceOf (".", false, false);
+        const auto extension = filename.contains (".") ? filename.fromLastOccurrenceOf (".", false, false)
+                                                       : String();
 
-        if (extension.isEmpty())
-        {
-            auto tokens = StringArray::fromTokens (filtersString, ";,", "\"'");
-            tokens.trim();
-            tokens.removeEmptyStrings();
+        if (! extension.isEmpty())
+            return extension;
 
-            if (tokens.size() == 1 && tokens[0].removeCharacters ("*.").isNotEmpty())
-                extension = tokens[0].fromFirstOccurrenceOf (".", false, false);
-        }
+        auto tokens = StringArray::fromTokens (filtersString, ";,", "\"'");
+        tokens.trim();
+        tokens.removeEmptyStrings();
+
+        if (tokens.size() == 1 && tokens[0].removeCharacters ("*.").isNotEmpty())
+            return tokens[0].fromFirstOccurrenceOf (".", false, false);
 
-        return extension;
+        return {};
     }
 
     //==============================================================================