Issue 1: Inconsistent file extensions in HTMLExporter and LatexExporter

Body:
The `HTMLExporter` and `LatexExporter` classes both inherit from `Exporter` and define a `file_extension` attribute. However, they are incorrectly set to "txt" instead of "html" and "tex" respectively. This will result in the converted files having the wrong extension, making them harder to open with the appropriate tools. The `Exporter` class defines it as "txt" but child classes should override this.