Code Quality: Enforce usage of `FileExtensionHelpers` to check file extensions across codebase

[Lamparter]

Description

There is a helper, FileExtensionHelpers, which is a unified static class for determining file type by extension. However, some parts of the codebase manually check file types without using that helper, e.g.:

Files/src/Files.App/ViewModels/UserControls/Previews/HtmlPreviewViewModel.cs

Lines 15 to 16 in 4e31a79

	public static bool ContainsExtension(string extension)
	=> extension is ".htm" or ".html" or ".svg";

Concerned code

~ Code search results ("ContainsExtension")

Gains

• Use of unified helper

Requirements

• Create new helper methods
• Replacing methods with existing ones

[yaira2]

All new code that we add uses the extension helper, there are only a few classes that don't already use it. It's not really worth the time and effort to spend time on these types of changes (it's usually something we prefer to do 'while we're in there') but if you'd like to make a PR, I will try to review it.