diff --git a/s3upload.js b/s3upload.js
index 7a30cb4..2d36f4f 100644
--- a/s3upload.js
+++ b/s3upload.js
@@ -175,7 +175,7 @@ S3Upload.prototype.uploadToS3 = function(file, signResult) {
         }.bind(this);
     }
 
-    var fileType = getFileMimeType(file);
+    var fileType = getFileMimeType(file) || 'application/octet-stream';
     var headers = {
       'content-type': fileType
     };