Do not use Enumerable methods on indexable collections

This rule flags the Enumerable LINQ method calls on collections of types that have equivalent but more efficient properties to fetch the same data.

.editorconfig

@@ -194,7 +194,6 @@ dotnet_diagnostic.CA1819.severity = suggestion
 dotnet_diagnostic.CA1822.severity = suggestion
 dotnet_diagnostic.CA1823.severity = suggestion
 dotnet_diagnostic.CA1824.severity = suggestion
-dotnet_diagnostic.CA1826.severity = suggestion
 dotnet_diagnostic.CA1827.severity = suggestion
 dotnet_diagnostic.CA1829.severity = suggestion
 dotnet_diagnostic.CA1834.severity = suggestion

src/Sonarr.Api.V3/System/Backup/BackupController.cs

@@ -93,7 +93,7 @@ namespace Sonarr.Api.V3.System.Backup
                 throw new BadRequestException("file must be provided");
             }
 
-            var file = files.First();
+            var file = files[0];
             var extension = Path.GetExtension(file.FileName);
 
             if (!ValidExtensions.Contains(extension))