File browser type icon should be bigger on files that do not support a preview

[QbieShay]

Godot version:
3.1 alpha

Issue description:
The type icon for the files in the inspector is generally small. This is good for files that support a preview ( scripts, tscn, models) but for other type of files (saved animations) it is very small and requires more effort to be seen

Proposal:
When a file format does not support a preview, make the file type icon big and in the middle of the file icon.