Add placeholder for original text/plain MIME body

### The problem

### Add placeholder for original `text/plain` MIME body

Would it be possible to add a separate placeholder that retrieves the **original `text/plain` MIME part** of an email directly from Thunderbird, when available?

Currently, with some Outlook-generated emails, `{%mail_text_body%}` appears to perform an HTML-to-text conversion. This can include Outlook/Word CSS (`@font-face`, `MsoNormal`, etc.) while also losing paragraph/line breaks.

For example, HTML containing:

```text
DMS could be XXXX
Server 2TB
AMC 5*500 = 2500+Tax
```

can become:

```text
@font-face ... MsoNormal ... DMS could be XXXXServer 2TBAMC 5*500 = 2500+Tax...
```

If Thunderbird exposes the original `text/plain` MIME part, a dedicated placeholder for it would be very useful for AI prompts.

**Suggested behavior:**

* Return the original `text/plain` MIME content without HTML conversion.
* Keep it separate from `{%mail_text_body%}` for backward compatibility.
* If no `text/plain` MIME part exists, return empty (or optionally fall back to the existing conversion).

This would reduce token usage and provide cleaner, correctly structured email content, especially for Outlook emails.

**Additional observation:** If I switch Thunderbird to View → Message Body As → Plain Text and then use the existing {%mail_text_body%} placeholder, ThunderAI correctly extracts the clean, properly formatted plain-text version of the email.

### Which Operative System are you using?

Windows 11

### Which version of Thunderbird are you using?

154.0

### Which version of ThunderAI has the issue?

4.1.1.0

### Which integration are you using?

ChatGPT Web Interface

### Anything in the Thunderbird console logs that might be useful?

```txt

```

### Additional information

_No response_

Navigation Menu

Add placeholder for original text/plain MIME body #869

The problem