diff --git a/src/payload/client.ts b/src/payload/client.ts
index 0f6dce9..2d2e821 100644
--- a/src/payload/client.ts
+++ b/src/payload/client.ts
@@ -100,9 +100,14 @@ class PayloadClient {
 
   async uploadMedia(file: Buffer, filename: string, mimeType: string, options: MediaUploadOptions): Promise<MediaDoc> {
     const formData = new FormData();
-    formData.append('alt', options.alt);
-    if (options.caption) formData.append('caption', options.caption);
-    formData.append('tenant', String(options.tenantId));
+
+    // Payload CMS requires document data as JSON in the _payload field
+    const payloadData: Record<string, unknown> = {
+      alt: options.alt,
+      tenant: options.tenantId,
+    };
+    if (options.caption) payloadData.caption = options.caption;
+    formData.append('_payload', JSON.stringify(payloadData));
 
     const arrayBuffer = new ArrayBuffer(file.byteLength);
     new Uint8Array(arrayBuffer).set(file);