Side Menu-Workflows Tree-Show Only json workflow items

### Prerequisites

- [x] I am running the latest version of ComfyUI
- [x] I have searched existing issues to make sure this isn't a duplicate
- [x] I have tested with all custom nodes disabled ([see how](https://docs.comfy.org/troubleshooting/custom-node-issues#step-1%3A-test-with-all-custom-nodes-disabled))

### What happened?

Viewing the saved user-workflows from the left panel tree, it includes non-json files. I often copy URL shortcut files with the workflows I save from 3rd part sources to reference back to the original among other text files too I keep for notes on sources. Non-JSON files should not be visible in the workflow tree view.

### Steps to Reproduce

1. Copy any non-JSON file into the ComfyUI/User/Default/Workflow directory
2. Start ComfyUI (or go to workflow menu and refresh)
3. View contents of workflows and you will see ALL unfiltered files (Only the JSON should be visible)

### How is this affecting you?

Visual/UI issue only

### ComfyUI Frontend Version

1.34.9

### Browser

Edge

### Console Errors

```javascript
No errors to report other than a simple filtering request on workflow tree list view
```

### Logs

```shell

```

### Additional Context

_No response_

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-7642-Side-Menu-Workflows-Tree-Show-Only-json-workflow-items-2ce6d73d36508160a7dfc5907089ba01) by [Unito](https://www.unito.io)