Issue 1: Inconsistent file extensions in HTMLExporter and LatexExporter #112
Description
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.