Code Quality: Use FileExtensionHelpers.IsRichTextFile for RTF (#18153)

Author: yair100

src/Files.App/ViewModels/UserControls/InfoPaneViewModel.cs

@@ -324,7 +324,7 @@ private async Task<UserControl> GetBuiltInPreviewControlAsync(ListedItem item, b
 				return new HtmlPreview(model);
 			}*/
 
-			if (RichTextPreviewViewModel.ContainsExtension(ext))
+			if (FileExtensionHelpers.IsRichTextFile(ext))
 			{
 				var model = new RichTextPreviewViewModel(item);
 				await model.LoadAsync();

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

@@ -12,9 +12,6 @@ public sealed partial class RichTextPreviewViewModel : BasePreviewModel
 
 		public RichTextPreviewViewModel(ListedItem item) : base(item) { }
 
-		public static bool ContainsExtension(string extension)
-			=> extension is ".rtf";
-
 		public async override Task<List<FileProperty>> LoadPreviewAndDetailsAsync()
 		{
 			Stream = await Item.ItemFile.OpenReadAsync();

src/Files.Shared/Helpers/FileExtensionHelpers.cs

@@ -327,5 +327,15 @@ public static bool IsTextFile(string? fileExtensionToCheck)
 		{
 			return HasExtension(fileExtensionToCheck, ".txt");
 		}
+
+		/// <summary>
+		/// Check if the file extension is a rich text file.
+		/// </summary>
+		/// <param name="fileExtensionToCheck"></param>
+		/// <returns><c>true</c> if the fileExtensionToCheck is a rich text file; otherwise, <c>false</c>.</returns>
+		public static bool IsRichTextFile(string? fileExtensionToCheck)
+		{
+			return HasExtension(fileExtensionToCheck, ".rtf");
+		}
 	}
 }