diff --git a/CHANGELOG.md b/CHANGELOG.md
index 011e1cbcc..d1ee8c744 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,16 @@
 -
 
 -->
+## 13.x.x (unreleased)
+
+### General
+-
+
+### Client
+-
+
+### Server
+- エクスポートデータの拡張子がunknownになる問題を修正
 
 ## 13.11.3
 
diff --git a/packages/backend/src/core/DriveService.ts b/packages/backend/src/core/DriveService.ts
index 65b4c24ca..7f66f1137 100644
--- a/packages/backend/src/core/DriveService.ts
+++ b/packages/backend/src/core/DriveService.ts
@@ -59,6 +59,8 @@ type AddFileArgs = {
 	uri?: string | null;
 	/** Mark file as sensitive */
 	sensitive?: boolean | null;
+	/** Extension to force */
+	ext?: string | null;
 
 	requestIp?: string | null;
 	requestHeaders?: Record<string, string> | null;
@@ -125,7 +127,7 @@ export class DriveService {
 	/***
 	 * Save file
 	 * @param path Path for original
-	 * @param name Name for original
+	 * @param name Name for original (should be extention corrected)
 	 * @param type Content-Type for original
 	 * @param hash Hash for original
 	 * @param size Size for original
@@ -151,7 +153,7 @@ export class DriveService {
 			}
 
 			// 拡張子からContent-Typeを設定してそうな挙動を示すオブジェクトストレージ (upcloud?) も存在するので、
-			// 許可されているファイル形式でしか拡張子をつけない
+			// 許可されているファイル形式でしかURLに拡張子をつけない
 			if (!FILE_TYPE_BROWSERSAFE.includes(type)) {
 				ext = '';
 			}
@@ -173,7 +175,7 @@ export class DriveService {
 			//#region Uploads
 			this.registerLogger.info(`uploading original: ${key}`);
 			const uploads = [
-				this.upload(key, fs.createReadStream(path), type, ext, name),
+				this.upload(key, fs.createReadStream(path), type, null, name),
 			];
 
 			if (alts.webpublic) {
@@ -189,7 +191,7 @@ export class DriveService {
 				thumbnailUrl = `${ baseUrl }/${ thumbnailKey }`;
 
 				this.registerLogger.info(`uploading thumbnail: ${thumbnailKey}`);
-				uploads.push(this.upload(thumbnailKey, alts.thumbnail.data, alts.thumbnail.type, alts.thumbnail.ext));
+				uploads.push(this.upload(thumbnailKey, alts.thumbnail.data, alts.thumbnail.type, alts.thumbnail.ext, `${name}.thumbnail`));
 			}
 
 			await Promise.all(uploads);
@@ -443,6 +445,7 @@ export class DriveService {
 		sensitive = null,
 		requestIp = null,
 		requestHeaders = null,
+		ext = null,
 	}: AddFileArgs): Promise<DriveFile> {
 		let skipNsfwCheck = false;
 		const instance = await this.metaService.fetch();
@@ -474,7 +477,7 @@ export class DriveService {
 			// DriveFile.nameは256文字, validateFileNameは200文字制限であるため、
 			// extを付加してデータベースの文字数制限に当たることはまずない
 			(name && this.driveFileEntityService.validateFileName(name)) ? name : 'untitled',
-			info.type.ext,
+			ext ?? info.type.ext,
 		);
 
 		if (user && !force) {
diff --git a/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts b/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
index a02000673..c7b54070d 100644
--- a/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportBlockingProcessorService.ts
@@ -106,7 +106,7 @@ export class ExportBlockingProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'blocking-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.csv';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'csv' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {
diff --git a/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts b/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
index daefcdf2f..f2f2383a8 100644
--- a/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportFavoritesProcessorService.ts
@@ -121,7 +121,7 @@ export class ExportFavoritesProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'favorites-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.json';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'json' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {
diff --git a/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts b/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
index 59443de57..fa9c1ac1e 100644
--- a/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportFollowingProcessorService.ts
@@ -110,7 +110,7 @@ export class ExportFollowingProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'following-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.csv';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'csv' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {
diff --git a/packages/backend/src/queue/processors/ExportMutingProcessorService.ts b/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
index a2a718b89..b14bf5f5b 100644
--- a/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportMutingProcessorService.ts
@@ -110,7 +110,7 @@ export class ExportMutingProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'mute-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.csv';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'csv' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {
diff --git a/packages/backend/src/queue/processors/ExportNotesProcessorService.ts b/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
index 1aa20d6f1..e4f12ad10 100644
--- a/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportNotesProcessorService.ts
@@ -117,7 +117,7 @@ export class ExportNotesProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'notes-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.json';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'json' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {
diff --git a/packages/backend/src/queue/processors/ExportUserListsProcessorService.ts b/packages/backend/src/queue/processors/ExportUserListsProcessorService.ts
index ce8ed2f5e..54bde4404 100644
--- a/packages/backend/src/queue/processors/ExportUserListsProcessorService.ts
+++ b/packages/backend/src/queue/processors/ExportUserListsProcessorService.ts
@@ -86,7 +86,7 @@ export class ExportUserListsProcessorService {
 			this.logger.succ(`Exported to: ${path}`);
 
 			const fileName = 'user-lists-' + dateFormat(new Date(), 'yyyy-MM-dd-HH-mm-ss') + '.csv';
-			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true });
+			const driveFile = await this.driveService.addFile({ user, path, name: fileName, force: true, ext: 'csv' });
 
 			this.logger.succ(`Exported to: ${driveFile.id}`);
 		} finally {