Before

After

(note: opened document gets closed after debugging)

How to build

To add in your dnSpy

build the dll from the source or from the release tab here: https://github.com/DeStilleGast/dnSpy-open-attached-process/releases
place the dll file in the directory where dnSpy.exe is located or in a subfolder called "Extensions"
(re)start dnSpy
enjoy

How to build

Follow instructions to add references from here https://github.com/0xd4d/dnSpy/wiki/Extensions
Add dnSpy.Contracts.Debugger.dll as reference (because of this, it might break)
build it
Copy "dnSpy show attach process.x.dll" to your dnSpy folder location (or a subfolder named "Extensions" and place the dll in there)
(re)start dnSpy
enjoy