diff --git a/packages/backend/src/server/api/mastodon/MastodonApiServerService.ts b/packages/backend/src/server/api/mastodon/MastodonApiServerService.ts
index ed56963156..36164e9682 100644
--- a/packages/backend/src/server/api/mastodon/MastodonApiServerService.ts
+++ b/packages/backend/src/server/api/mastodon/MastodonApiServerService.ts
@@ -1,5 +1,6 @@
 import { Inject, Injectable } from '@nestjs/common';
 import megalodon, { Entity, MegalodonInterface } from 'megalodon';
+import multipart from '@fastify/multipart';
 import { IsNull } from 'typeorm';
 import multer from 'fastify-multer';
 import type { UsersRepository } from '@/models/_.js';
@@ -42,6 +43,13 @@ export class MastodonApiServerService {
 
 		fastify.register(multer.contentParser);
 
+		fastify.register(multipart, {
+			limits: {
+				fileSize: this.config.maxFileSize ?? 262144000,
+				files: 1,
+			},
+		});
+
 		fastify.get('/v1/custom_emojis', async (_request, reply) => {
 			const BASE_URL = `${_request.protocol}://${_request.hostname}`;
 			const accessTokens = _request.headers.authorization;