text: Title: Markdown cells are saved as Python files for the external editor
  Body: First of all, thank you for making this program!
  
  ## Description of the issue
  
  When editing a Markdown cell through the external editor,
  the temporary file is saved as `/tmp/[...].py` instead of `/tmp/[...].md`
  (with the Python source file extension instead of Markdown).
  
  This causes the external text editor to use Python syntax highlighting instead
  of Markdown.
  
  ## How to reproduce
  
  1. Launch `euporie-notebook`,
  2. Create a new notebook with `<C-n>`,
  3. Set the current cell format to Markdown with `<m>`,
  4. Edit the cell with the external editor with `<e>`,
  5. Notice that the temporary text file is `/tmp/[...].py`.
  
  ## Version information
  
  ```
  ❯ euporie-notebook --version
  euporie-notebook 2.8.4
  ```
  