What happened:

The icon for the .USS and .uXML file appears as a unity icon with grey background whereas it should appear as a white page with USS written on it in blue.

Steps to reproduce:

1. Create a text file and change the extension to .USS or .uXML using any text editor.
2. Place the file in the resources folder in the editor.
3. Observe that unity icon with grey background appears for the .USS file and .uXML file.

Observed Result:
The icon for the .USS file and .uXML file appears to be the unity icon/symbol with a grey background.

Expected Result:
The icon for the .USS file should a white page with USS written in blue on it.

Frequency:
Always

Bug Reproducable on :
Unity 5.5.0, 5.6.0, 2017.1.0f1, 2017.2.0f3, 2017.3.0b1, 2017.3.0b10

Additional Note:
On unity version before 5.6, the icon appears to be a white page but the text .USS is missing from it.

Environment:
Unity 2017.3.0b10
Occuring on Windows and Mac.