New: Importing `sup` files as subtitles

Towards #10412

src/NzbDrone.Core/Extras/Subtitles/SubtitleFileExtensions.cs

@@ -5,11 +5,7 @@ namespace NzbDrone.Core.Extras.Subtitles
 {
     public static class SubtitleFileExtensions
     {
-        private static HashSet<string> _fileExtensions;
-
-        static SubtitleFileExtensions()
-        {
-            _fileExtensions = new HashSet<string>(StringComparer.OrdinalIgnoreCase)
+        public static HashSet<string> Extensions => new (StringComparer.OrdinalIgnoreCase)
         {
             ".aqt",
             ".ass",
@@ -21,6 +17,7 @@ namespace NzbDrone.Core.Extras.Subtitles
             ".srt",
             ".ssa",
             ".sub",
+            ".sup",
             ".txt",
             ".utf",
             ".utf8",
@@ -28,7 +25,4 @@ namespace NzbDrone.Core.Extras.Subtitles
             ".vtt"
         };
     }
-
-        public static HashSet<string> Extensions => _fileExtensions;
-    }
 }