Rename extension to UPPERCASE leads to subgraph failure

Steps to reproduce:

1.  Create subgraph operator in vfx
2.  Open location folder in explorer and rename .vfxoperator to .VFXOPERATOR
3.  Go back to Unity project browser

Actual results: 
The icons change to VFX and the file can't be open

Expected results: 
The case of extension shouldn't have impact on behavior and file type detection

Reproducible with versions: 6000.6a1

Not reproducible with versions: N/A

Can’t test with versions: N/A

Tested on (OS): Windows

Notes:

•  This is actually related to user suggestion at https://github.com/Unity-Technologies/Graphics/pull/8271