need format conversion

[iTyler]

I have some notes from the past, including math symbols.
Now I want to change them from txt to md.
The first time I tried, I renamed the file to .md and opened it, then I found *、_ are gone and normal text turn to italic. That's why I came here searching plugin.
When I used this, I can open .txt file, and display them as markdwon, even edit it. You really do a wonderful job. But my problem is not solved. It's not that complicated, I can replace it myself, like * to \*, _ to \_, some code parts add ```. But here's the thing, no such plugin. No choice.