Textbundle files are not simple files, but directories that are disguised as files, containing all the necessary information and additional files, such as images (Attachments).
Textpack files are structured like Textbundle files, but additionally compressed as a zip file, which makes it easier to share these files (for example, as an email attachment).
Textpack files can also be opened manually by renaming the file extension from Textpack to Zip. After that, these files can be unpacked just like traditional zip files.
More information about the Textbundle format can be found on the official Textbundle project page.