diff --git a/packages/misskey-js/src/api.ts b/packages/misskey-js/src/api.ts
index e663d712a7..0d1641090e 100644
--- a/packages/misskey-js/src/api.ts
+++ b/packages/misskey-js/src/api.ts
@@ -106,7 +106,7 @@ export class APIClient {
 			this.fetch(`${this.origin}/api/${endpoint}`, {
 				method: 'POST',
 				body: payload,
-				headers: {
+				headers: mediaType === 'multipart/form-data' ? {} : {
 					'Content-Type': mediaType,
 				},
 				credentials: 'omit',
diff --git a/packages/misskey-js/test/api.ts b/packages/misskey-js/test/api.ts
index 1a7574de25..b8ad2225c8 100644
--- a/packages/misskey-js/test/api.ts
+++ b/packages/misskey-js/test/api.ts
@@ -94,11 +94,7 @@ describe('API', () => {
 		fetchMock.resetMocks();
 		fetchMock.mockResponse(async (req) => {
 			if (req.method == 'POST' && req.url == 'https://misskey.test/api/drive/files/create') {
-				if (req.headers.get('Content-Type')?.includes('multipart/form-data')) {
-					return JSON.stringify({ id: 'foo' });
-				} else {
-					return { status: 400 };
-				}
+				return JSON.stringify({ id: 'foo' });
 			} else {
 				return { status: 404 };
 			}
@@ -123,7 +119,7 @@ describe('API', () => {
 		expect(getFetchCall(fetchMock.mock.calls[0])).toEqual({
 			url: 'https://misskey.test/api/drive/files/create',
 			method: 'POST',
-			contentType: 'multipart/form-data',
+			contentType: undefined,
 			body: {
 				i: 'TOKEN',
 				file: testFile,