diff --git a/packages/backend/src/misc/correct-filename.ts b/packages/backend/src/misc/correct-filename.ts
index 9a1298b957..34cb458a2c 100644
--- a/packages/backend/src/misc/correct-filename.ts
+++ b/packages/backend/src/misc/correct-filename.ts
@@ -7,16 +7,16 @@
  * Array.includes()よりSet.has()の方が高速
  */
 const targetExtsToSkip = new Set([
-	'gz',
-	'tar',
-	'tgz',
-	'bz2',
-	'xz',
-	'zip',
-	'7z',
+	'.gz',
+	'.tar',
+	'.tgz',
+	'.bz2',
+	'.xz',
+	'.zip',
+	'.7z',
 ]);
 
-const extRegExp = /\.([0-9a-zA-Z]+$)/i;
+const extRegExp = /\.[0-9a-zA-Z]+$/i;
 
 /**
  * 与えられた拡張子とファイル名が一致しているかどうかを確認し、
@@ -25,28 +25,28 @@ const extRegExp = /\.([0-9a-zA-Z]+$)/i;
  * extはfile-typeのextを想定
  */
 export function correctFilename(filename: string, ext: string | null) {
-	const dotExt = ext ? ext.startsWith('.') ? ext : `.${ext}` : '.unknown';
+	const dotExt = ext ? ext[0] === '.' ? ext : `.${ext}` : '.unknown';
 
 	const match = extRegExp.exec(filename);
-	if (!match || !match[1]) {
+	if (!match || !match[0]) {
 		// filenameが拡張子を持っていない場合は拡張子をつける
 		return `${filename}${dotExt}`;
 	}
 
-	const filenameExt = match[1].toLowerCase();
+	const filenameExt = match[0].toLowerCase();
 	if (
 		// 未知のファイル形式かつ拡張子がある場合は何もしない
 		ext === null ||
 		// 拡張子が一致している場合は何もしない
-		filenameExt === ext ||
+		filenameExt === dotExt ||
 
 		// jpeg, tiffを同一視
-		ext === 'jpg' && filenameExt === 'jpeg' ||
-		ext === 'tif' && filenameExt === 'tiff' ||
+		dotExt === '.jpg' && filenameExt === '.jpeg' ||
+		dotExt === '.tif' && filenameExt === '.tiff' ||
 
 		// 圧縮形式っぽければ下手に拡張子を変えない
 		// https://github.com/misskey-dev/misskey/issues/11482
-		targetExtsToSkip.has(ext)
+		targetExtsToSkip.has(dotExt)
 	) {
 		return filename;
 	}
diff --git a/packages/backend/test/unit/misc/correct-filename.ts b/packages/backend/test/unit/misc/correct-filename.ts
index 77498ec3dc..28cdd382dd 100644
--- a/packages/backend/test/unit/misc/correct-filename.ts
+++ b/packages/backend/test/unit/misc/correct-filename.ts
@@ -39,4 +39,7 @@ describe(correctFilename, () => {
     it('unknown', () => {
         expect(correctFilename('test.hoge', null)).toBe('test.hoge');
     });
+	test('non ascii with space', () => {
+		expect(correctFilename('ファイル 名前', 'jpg')).toBe('ファイル 名前.jpg');
+	});
 });
diff --git a/packages/backend/test/unit/misc/others.ts b/packages/backend/test/unit/misc/others.ts
index e2e484dfd7..b16d26d866 100644
--- a/packages/backend/test/unit/misc/others.ts
+++ b/packages/backend/test/unit/misc/others.ts
@@ -5,7 +5,6 @@
 
 import { describe, test, expect } from '@jest/globals';
 import { contentDisposition } from '@/misc/content-disposition.js';
-import { correctFilename } from '@/misc/correct-filename.js';
 
 describe('misc:content-disposition', () => {
 	test('inline', () => {
@@ -18,30 +17,3 @@ describe('misc:content-disposition', () => {
 		expect(contentDisposition('attachment', 'ファイル名')).toBe('attachment; filename=\"_____\"; filename*=UTF-8\'\'%E3%83%95%E3%82%A1%E3%82%A4%E3%83%AB%E5%90%8D');
 	});
 });
-
-describe('misc:correct-filename', () => {
-	test('simple', () => {
-		expect(correctFilename('filename', 'jpg')).toBe('filename.jpg');
-	});
-	test('with same ext', () => {
-		expect(correctFilename('filename.jpg', 'jpg')).toBe('filename.jpg');
-	});
-	test('.ext', () => {
-		expect(correctFilename('filename.jpg', '.jpg')).toBe('filename.jpg');
-	});
-	test('with different ext', () => {
-		expect(correctFilename('filename.webp', 'jpg')).toBe('filename.webp.jpg');
-	});
-	test('non ascii with space', () => {
-		expect(correctFilename('ファイル 名前', 'jpg')).toBe('ファイル 名前.jpg');
-	});
-	test('jpeg', () => {
-		expect(correctFilename('filename.jpeg', 'jpg')).toBe('filename.jpeg');
-	});
-	test('tiff', () => {
-		expect(correctFilename('filename.tiff', 'tif')).toBe('filename.tiff');
-	});
-	test('null ext', () => {
-		expect(correctFilename('filename', null)).toBe('filename.unknown');
-	});
-});