diff --git a/index.js b/index.js
index 675016c..6a0d9df 100644
--- a/index.js
+++ b/index.js
@@ -214,7 +214,7 @@ var SecureFile = /** @class */ (function () {
                     case 0:
                         params = { type: type, name: name };
                         if (preview) {
-                            params.preview = preview;
+                            params.preview = preview.toString("base64");
                         }
                         if (folder) {
                             params.folder = folder;
@@ -285,7 +285,7 @@ var SecureFile = /** @class */ (function () {
                     case 0:
                         put = {};
                         if (preview)
-                            put.preview = preview;
+                            put.preview = preview.toString("base64");
                         if (meta)
                             put.meta = meta;
                         return [4 /*yield*/, this.makeRequest("/files/" + id, "PUT", put, data, this.jwt_enabled)];
diff --git a/index.ts b/index.ts
index 4a2565b..fd9efad 100644
--- a/index.ts
+++ b/index.ts
@@ -117,7 +117,7 @@ export default class SecureFile {
    async create(name: string, data: Buffer, type: "text" | "binary", folder?: string, preview?: Buffer, meta?: string): Promise<File> {
       let params: any = { type: type, name: name };
       if (preview) {
-         params.preview = preview;
+         params.preview = preview.toString("base64");
       }
       if (folder) {
          params.folder = folder;
@@ -153,7 +153,7 @@ export default class SecureFile {
 
    async update(id: string, data: Buffer, preview?: Buffer, meta?: string): Promise<File> {
       let put: any = {};
-      if (preview) put.preview = preview;
+      if (preview) put.preview = preview.toString("base64");
       if (meta) put.meta = meta;
       let res = await this.makeRequest("/files/" + id, "PUT", put, data, this.jwt_enabled);
       statusParser(res);