The Textpack format is part of the TextBundle standard (http://textbundle.org/). It's a container that allows you to store markdown documents together with images as a single file.
This is a zip file, containing a markdown file with all the images used in the document.
If you don't have an application that supports this format, you can simply extract the file with any data compression application. For example:
creating: Textbundle Example.textbundle/
inflating: Textbundle Example.textbundle/text.markdown
inflating: Textbundle Example.textbundle/info.json
creating: Textbundle Example.textbundle/assets/
inflating: Textbundle Example.textbundle/assets/textbundle.png
Textpack also includes a standard to transfer additional information - to open up new possibilites for future integration.