Export dialog - Missing overwrite warning when file extension missing

[Dstruct]

XnView 2.50.4

Precondition: File "Test.png" already exists

1. add Clipboard as new picture (CTRL+SHIFT+V)
2. open export dialog (CTRL+ALT+S) with PNG tab
3. click "Save"
4. enter "Test" as filename
5. click "Save"

Result: Existing file "Test.png" will be overwritten without warning. This bug doesn't occur when you enter "Test.png" as filename in step 4.

Expected: Overwrite warning dialog getting displayed.