How to reproduce:
1. Create a new project with Scriptable Render Pipeline template (i.e. URP)
2. In the Project Window, create a new Shader Graph
3. In the Shader Graph, create a "Custom Function" node
4. In the Graph Inspector, view the available "Source" files

Expected result: HLSL and CGINC files can be selected
Actual result: Only TXT files can be selected and they throw errors

Reproducible with: 12.0.0 (2021.2.0a8, 2021.2.0a12)
Not reproducible with: 7.5.1 (2019.4.23f1), 10.4.0 (2020.3.3f1), 11.0.0 (2021.1.1f1), 12.0.0 (2021.2.0a7)