Creating a text or Markdown file with a `.png` extension results in an invalid image file

## Description

When creating a new **Plain Text** or **Markdown** file, it is possible to change the file extension to `.png` before saving.

For example:
1. Create a new **Plain Text** or **Markdown** file.
2. Rename `text.txt` or `text.md` to `text.png`.
3. Enter text content and save the file.

After saving, the file is treated as an image instead of a text file. Opening it results in the following error:

> Failed to load "text.png"

The file contains text content but is handled as a PNG image based only on its extension.

## Expected Result

- The file type should be validated before saving.
- Users should not be able to save a text or Markdown file with an incompatible `.png` extension, or the application should warn the user that the file content does not match the selected extension.

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### Screen-cast

https://github.com/user-attachments/assets/74720ac5-fa77-4520-936a-22eb710373c3

Reproduce the issue by creating a Plain Text or Markdown file, changing its extension to .png, entering text, and saving it. Trace the save and file-type handling from that workflow; done means incompatible content and extension are rejected or clearly warned about, and the resulting file is not treated as a PNG.

Written by the indexing model from the issue text.