Reproduction steps:
1. Open the attached project "UIBuilderUSSDialogue.zip"
2. Open the UI Builder window
3. Press the "Add" button on the top left side of UI Builder window to add an existing StyleSheet
4. In the file browser bottom right side select "Any Files"
5. Select any file type and add it as a USS StyleSheet

Expected result: dialogue box displays the wrong asset being loaded "Asset at path Assets/Scene.meta is not a StyleSheet"
Actual result: dialogue box displays "Asset at path {ussPath} is not a StyleSheet" (See attached "StyleSheet.mp4")

Reproduces on: UI Builder - preview 0.11.0 (2019.3.16f1), UI Builder preview.1 - 1.0.0 (2020.2.0a16)
Could not test UI Builder preview - 0.10.2 and below due to no option to add an existing StyleSheet
Could not test 2018.4 (No package)

Notes:
-in UI Builder preview - 0.11.0 to add an existing StyleSheet you need to right-click in the StyleSheets window to see the option appear