diff --git a/src/compile-dts.ts b/src/compile-dts.ts
index f1ca6cd..2e26682 100644
--- a/src/compile-dts.ts
+++ b/src/compile-dts.ts
@@ -1,4 +1,3 @@
-import * as path from 'path';
 import * as ts from 'typescript';
 
 import { verboseLog, warnLog } from './logger';
@@ -12,6 +11,25 @@ export interface CompileDtsResult {
 	rootFilesRemapping: Map<string, string>;
 }
 
+const declarationExtsRemapping: Record<string, ts.Extension> = {
+	[ts.Extension.Js]: ts.Extension.Js,
+	[ts.Extension.Jsx]: ts.Extension.Jsx,
+	[ts.Extension.Json]: ts.Extension.Json,
+	[ts.Extension.TsBuildInfo]: ts.Extension.TsBuildInfo,
+	[ts.Extension.Mjs]: ts.Extension.Mjs,
+	[ts.Extension.Cjs]: ts.Extension.Cjs,
+
+	[ts.Extension.Ts]: ts.Extension.Dts,
+	[ts.Extension.Tsx]: ts.Extension.Dts,
+	[ts.Extension.Dts]: ts.Extension.Dts,
+
+	[ts.Extension.Mts]: ts.Extension.Dmts,
+	[ts.Extension.Dmts]: ts.Extension.Dmts,
+
+	[ts.Extension.Cts]: ts.Extension.Dcts,
+	[ts.Extension.Dcts]: ts.Extension.Dcts,
+} satisfies Record<ts.Extension, ts.Extension>;
+
 export function compileDts(rootFiles: readonly string[], preferredConfigPath?: string, followSymlinks: boolean = true): CompileDtsResult {
 	const compilerOptions = getCompilerOptions(rootFiles, preferredConfigPath);
 
@@ -41,13 +59,16 @@ export function compileDts(rootFiles: readonly string[], preferredConfigPath?: s
 	host.resolveModuleNameLiterals = (moduleLiterals: readonly ts.StringLiteralLike[], containingFile: string): ts.ResolvedModuleWithFailedLookupLocations[] => {
 		return moduleLiterals.map((moduleLiteral: ts.StringLiteralLike): ts.ResolvedModuleWithFailedLookupLocations => {
 			const resolvedModule = ts.resolveModuleName(moduleLiteral.text, containingFile, compilerOptions, host).resolvedModule;
-			// eslint-disable-next-line @typescript-eslint/no-unsafe-enum-comparison
-			if (resolvedModule && !resolvedModule.isExternalLibraryImport && resolvedModule.extension !== ts.Extension.Dts) {
-				resolvedModule.extension = ts.Extension.Dts;
+			if (resolvedModule && !resolvedModule.isExternalLibraryImport) {
+				const newExt = declarationExtsRemapping[resolvedModule.extension];
 
-				verboseLog(`Change module from .ts to .d.ts: ${resolvedModule.resolvedFileName}`);
+				// eslint-disable-next-line @typescript-eslint/no-unsafe-enum-comparison
+				if (newExt !== resolvedModule.extension) {
+					verboseLog(`Changing module from ${resolvedModule.extension} to ${newExt} for ${resolvedModule.resolvedFileName}`);
 
-				resolvedModule.resolvedFileName = changeExtensionToDts(resolvedModule.resolvedFileName);
+					resolvedModule.extension = newExt;
+					resolvedModule.resolvedFileName = changeExtensionToDts(resolvedModule.resolvedFileName);
+				}
 			}
 
 			return { resolvedModule };
@@ -82,13 +103,28 @@ export function compileDts(rootFiles: readonly string[], preferredConfigPath?: s
 }
 
 function changeExtensionToDts(fileName: string): string {
-	if (fileName.slice(-5) === '.d.ts') {
+	let ext: ts.Extension | undefined;
+
+	// `path.extname` doesn't handle `.d.ts` cases (it returns `.ts` instead of `.d.ts`)
+	if (fileName.endsWith(ts.Extension.Dts)) {
+		return fileName;
+	}
+
+	if (fileName.endsWith(ts.Extension.Cts)) {
+		ext = ts.Extension.Cts;
+	} else if (fileName.endsWith(ts.Extension.Mts)) {
+		ext = ts.Extension.Mts;
+	} else if (fileName.endsWith(ts.Extension.Ts)) {
+		ext = ts.Extension.Ts;
+	} else if (fileName.endsWith(ts.Extension.Tsx)) {
+		ext = ts.Extension.Tsx;
+	}
+
+	if (ext === undefined) {
 		return fileName;
 	}
 
-	// .ts, .tsx
-	const ext = path.extname(fileName);
-	return fileName.slice(0, -ext.length) + '.d.ts';
+	return fileName.slice(0, -ext.length) + declarationExtsRemapping[ext];
 }
 
 /**
diff --git a/tests/e2e/all-test-cases.ts b/tests/e2e/all-test-cases.ts
index e8c2652..4236f75 100644
--- a/tests/e2e/all-test-cases.ts
+++ b/tests/e2e/all-test-cases.ts
@@ -28,6 +28,30 @@ function prepareString(str: string): string {
 	return str.trim().replace(/\r\n/g, '\n');
 }
 
+function findInputFile(testCaseDir: string): string {
+	const tsFilePath = path.join(testCaseDir, 'input.ts');
+	if (fs.existsSync(tsFilePath)) {
+		return tsFilePath;
+	}
+
+	const dtsFilePath = path.join(testCaseDir, 'input.d.ts');
+	if (fs.existsSync(dtsFilePath)) {
+		return dtsFilePath;
+	}
+
+	const mtsFilePath = path.join(testCaseDir, 'input.mts');
+	if (fs.existsSync(mtsFilePath)) {
+		return mtsFilePath;
+	}
+
+	const ctsFilePath = path.join(testCaseDir, 'input.cts');
+	if (fs.existsSync(ctsFilePath)) {
+		return ctsFilePath;
+	}
+
+	throw new Error(`Cannot find input file in ${testCaseDir}`);
+}
+
 function getTestCases(): TestCase[] {
 	return fs.readdirSync(testCasesDir)
 		.filter((filePath: string) => {
@@ -35,14 +59,10 @@ function getTestCases(): TestCase[] {
 		})
 		.map((directoryName: string) => {
 			const testCaseDir = path.resolve(testCasesDir, directoryName);
-			const outputFileName = path.resolve(testCaseDir, 'output.d.ts');
-
-			const tsFilePath = path.relative(process.cwd(), path.resolve(testCaseDir, 'input.ts'));
-			const dtsFilePath = path.relative(process.cwd(), path.resolve(testCaseDir, 'input.d.ts'));
 
-			const inputFileName = fs.existsSync(tsFilePath) ? tsFilePath : dtsFilePath;
+			const inputFileName = findInputFile(testCaseDir);
 
-			assert(fs.existsSync(inputFileName), `Input file doesn't exist for ${directoryName}`);
+			const outputFileName = path.resolve(testCaseDir, 'output.d.ts');
 			assert(fs.existsSync(outputFileName), `Output file doesn't exist for ${directoryName}`);
 
 			const result: TestCase = {
diff --git a/tests/e2e/test-cases/cts-extension/config.ts b/tests/e2e/test-cases/cts-extension/config.ts
new file mode 100644
index 0000000..f1fb1a8
--- /dev/null
+++ b/tests/e2e/test-cases/cts-extension/config.ts
@@ -0,0 +1,5 @@
+import { TestCaseConfig } from '../../test-cases/test-case-config';
+
+const config: TestCaseConfig = {};
+
+export = config;
diff --git a/tests/e2e/test-cases/cts-extension/decl.d.cts b/tests/e2e/test-cases/cts-extension/decl.d.cts
new file mode 100644
index 0000000..32c01d6
--- /dev/null
+++ b/tests/e2e/test-cases/cts-extension/decl.d.cts
@@ -0,0 +1 @@
+export type Decl = string;
diff --git a/tests/e2e/test-cases/cts-extension/file.cts b/tests/e2e/test-cases/cts-extension/file.cts
new file mode 100644
index 0000000..ef1ff89
--- /dev/null
+++ b/tests/e2e/test-cases/cts-extension/file.cts
@@ -0,0 +1 @@
+export interface Interface {}
diff --git a/tests/e2e/test-cases/cts-extension/input.cts b/tests/e2e/test-cases/cts-extension/input.cts
new file mode 100644
index 0000000..32954d0
--- /dev/null
+++ b/tests/e2e/test-cases/cts-extension/input.cts
@@ -0,0 +1,6 @@
+import { Interface } from './file.cjs';
+import { Decl } from './decl.cjs';
+
+export interface ExportedInterface extends Interface {
+	foo: Decl;
+}
diff --git a/tests/e2e/test-cases/cts-extension/output.d.ts b/tests/e2e/test-cases/cts-extension/output.d.ts
new file mode 100644
index 0000000..2507e88
--- /dev/null
+++ b/tests/e2e/test-cases/cts-extension/output.d.ts
@@ -0,0 +1,8 @@
+export interface Interface {
+}
+export type Decl = string;
+export interface ExportedInterface extends Interface {
+	foo: Decl;
+}
+
+export {};
diff --git a/tests/e2e/test-cases/mts-extension/config.ts b/tests/e2e/test-cases/mts-extension/config.ts
new file mode 100644
index 0000000..f1fb1a8
--- /dev/null
+++ b/tests/e2e/test-cases/mts-extension/config.ts
@@ -0,0 +1,5 @@
+import { TestCaseConfig } from '../../test-cases/test-case-config';
+
+const config: TestCaseConfig = {};
+
+export = config;
diff --git a/tests/e2e/test-cases/mts-extension/decl.d.mts b/tests/e2e/test-cases/mts-extension/decl.d.mts
new file mode 100644
index 0000000..32c01d6
--- /dev/null
+++ b/tests/e2e/test-cases/mts-extension/decl.d.mts
@@ -0,0 +1 @@
+export type Decl = string;
diff --git a/tests/e2e/test-cases/mts-extension/file.mts b/tests/e2e/test-cases/mts-extension/file.mts
new file mode 100644
index 0000000..ef1ff89
--- /dev/null
+++ b/tests/e2e/test-cases/mts-extension/file.mts
@@ -0,0 +1 @@
+export interface Interface {}
diff --git a/tests/e2e/test-cases/mts-extension/input.mts b/tests/e2e/test-cases/mts-extension/input.mts
new file mode 100644
index 0000000..6f75406
--- /dev/null
+++ b/tests/e2e/test-cases/mts-extension/input.mts
@@ -0,0 +1,6 @@
+import { Interface } from './file.mjs';
+import { Decl } from './decl.mjs';
+
+export interface ExportedInterface extends Interface {
+	foo: Decl;
+}
diff --git a/tests/e2e/test-cases/mts-extension/output.d.ts b/tests/e2e/test-cases/mts-extension/output.d.ts
new file mode 100644
index 0000000..2507e88
--- /dev/null
+++ b/tests/e2e/test-cases/mts-extension/output.d.ts
@@ -0,0 +1,8 @@
+export interface Interface {
+}
+export type Decl = string;
+export interface ExportedInterface extends Interface {
+	foo: Decl;
+}
+
+export {};