Clarify `ConcreteAssetLibraryHelper::file()`

Clarify `ConcreteAssetLibraryHelper::file()`

- Validate `$_POST[$postname]` is integer
- Use `$_POST[$postname]` if valid File ID
- Use `$bf` parameter if not
Merge pull request #1169 from mkly/clarify_concrete_asset_library_helper

Clarify `ConcreteAssetLibraryHelper::file()`