Failed to export Insomnia v5

### Expected Behavior

Hopefully, the multi-part request can be exported successfully.

### Actual Behavior

The body is set to Multipart, the parameter is multi-line text, the text type is selected as JSON, and the export format is set to Insomnia V5. Exporting will fail.

Insomnia info:
Version: Insomnia 12.5.0
Build date: 4/2/2026
OS: Darwin arm64 25.4.0
Electron: 41.0.3
Node: 24.14.0
Node ABI: 145
V8: 14.6.650202-electron.0
Architecture: arm64

### Reproduction Steps

For example：
curl --request POST \
--url https://github.com/Kong/insomnia \
--header 'Content-Type: multipart/form-data' \
--header 'User-Agent: insomnia/12.5.0' \
--form 'data={
"userName": "Email hzs"
}' \
--form headImg=@/Users/headImg.jpeg

### Is there an existing issue for this?

- [x] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Which sync method do you use?

- [ ] Git sync.
- [ ] Insomnia Cloud sync.
- [x] Local only

### Additional Information

_No response_

### Insomnia Version

12.5.0

### What operating system are you using?

macOS

### Operating System Version

MacOS 26.4

### Installation method

apt

### Last Known Working Insomnia version

_No response_

Start by reproducing the multipart request export in Insomnia 12.5.0 with a multi-line JSON text parameter and the Insomnia V5 format, then trace the export path that handles multipart bodies. Done means the request exports successfully without failure and preserves the multipart fields and JSON text.

Written by the indexing model from the issue text.