Is there a file extension (and major mode) for monospaced unicode "art" ?

I derive a lot of benefit from mind-mapping in the style shown below. I usually do this in .txt files. But is there some generally accepted text file extension specifically for monospace art?

<span style="color:#323232;">  ┌──────────────────┐           
</span><span style="color:#323232;"> ┌┤ Current Projects ├─────────┐ ┌─────────────────────────────────────────────────────────────────────┐
</span><span style="color:#323232;"> │└──────────────────┘         ├─┘ ┌──────────────────┐                                                │
</span><span style="color:#323232;"> │                             │   │ Possessions List │────────────┐                                   │ 
</span><span style="color:#323232;"> │ □ Forever Park              │   └──────────────────┘            │    ┌──────────────┐               │
</span><span style="color:#323232;"> │ └──────────────             │    │                              │   ┌┤ Forever Park ├────────────┐  │
</span><span style="color:#323232;"> │                             │    │  ░ Some reboxing would be    │   │└──────────────┘            │  │
</span><span style="color:#323232;"> │ □ Possessions List          │    │  ░   nice                    │   │                            │  │
</span><span style="color:#323232;"> │ └───────────────────        │    │                              │   │ ░ Not much. Just keep      │  │
</span><span style="color:#323232;"> │                             │    └──────────────────────────────┘   │ ░   plugging away at that  │  │
</span><span style="color:#323232;"> │ □ Healthcare Application    │    ┌────────────────────────┐         │ ░   outline.               │  │
</span><span style="color:#323232;"> │ └─────────────────────────  │    │ Healthcare Application │───────┐ │                            │  │
</span><span style="color:#323232;"> │                             │    └────────────────────────┘       │ └────────────────────────────┘  │
</span><span style="color:#323232;"> │ □ Tapstry Map               │     │                               │                                 │
</span><span style="color:#323232;"> │ └──────────────             │     │ ░ Previous healthcare expires │                                 │
</span><span style="color:#323232;"> │                             │     │ ░   Sunday, April 30th        │                                 │
</span><span style="color:#323232;"> └┬────────────────────────────┘     │                               │                                 │
</span><span style="color:#323232;"> ┌┘                                  └───────────────────────────────┘                                 │
</span><span style="color:#323232;"> │                                                                                                     │
</span><span style="color:#323232;"> │     ┌─────────────┐                                                                                 │
</span><span style="color:#323232;"> │    ┌┤ Tapstry Map ├─────────────────────────────────────────┐                                       │
</span><span style="color:#323232;"> │    │└─────────────┘                                         │                                       │
</span><span style="color:#323232;"> │    │                                                        │                                       │
</span><span style="color:#323232;"> │    │  Explore:                                              │                                       │
</span><span style="color:#323232;"> │    │                                                        │                                       │
</span><span style="color:#323232;"> │    │   ░  line endpoints                                    │                                       │
</span><span style="color:#323232;"> │    │   ░  text box wrapping                                 │                                       │
</span><span style="color:#323232;"> │    │   ░  google sheets and csv files with real datasets    │                                       │
</span><span style="color:#323232;"> │    │   ░  d3 pointer events so I can see mouse coordinates  │                                       │
</span><span style="color:#323232;"> │    │                                                        │                                       │
</span><span style="color:#323232;"> │    └────────────────────────────────────────────────────────┘                                       │
</span><span style="color:#323232;"> │                                                                                                     │
</span><span style="color:#323232;"> └─────────────────────────────────────────────────────────────────────────────────────────────────────┘
</span><span style="color:#323232;">
</span>