Addressed PR comments

src/compiler/core.ts

@@ -2677,7 +2677,7 @@ namespace ts {
 
         const extensions = [
             ...needJsExtensions ? allSupportedExtensions : supportedTypeScriptExtensions,
-            ...extraFileExtensions.filter(x => x.scriptKind === ScriptKind.Deferred || needJsExtensions && isJavaScriptLike(x.scriptKind)).map(x => x.extension)
+            ...mapDefined(extraFileExtensions, x => x.scriptKind === ScriptKind.Deferred || needJsExtensions && isJavaScriptLike(x.scriptKind) ? x.extension : undefined)
         ];
 
         return deduplicate(extensions, equateStringsCaseSensitive, compareStringsCaseSensitive);

src/compiler/types.ts

@@ -4066,6 +4066,8 @@ namespace ts {
         Prototype,
     }
 
+    export type JsFileExtensionInfo = FileExtensionInfo;
+
     export interface FileExtensionInfo {
         extension: string;
         isMixedContent: boolean;

tests/baselines/reference/api/tsserverlibrary.d.ts

@@ -2266,6 +2266,7 @@ declare namespace ts {
         AlwaysStrict = 64,
         PriorityImpliesCombination = 28
     }
+    type JsFileExtensionInfo = FileExtensionInfo;
     interface FileExtensionInfo {
         extension: string;
         isMixedContent: boolean;

tests/baselines/reference/api/typescript.d.ts

@@ -2266,6 +2266,7 @@ declare namespace ts {
         AlwaysStrict = 64,
         PriorityImpliesCombination = 28
     }
+    type JsFileExtensionInfo = FileExtensionInfo;
     interface FileExtensionInfo {
         extension: string;
         isMixedContent: boolean;