Uploading Files with Ajax XMLHttpRequest: Troubleshooting the "No Multipart Boundary Found" Error

When attempting to upload a file using XMLHttpRequest, you may encounter an error like "The request was rejected because no multipart boundary was found." This issue arises when the file is not sent correctly as a multipart/form-data request.

One common mistake is attaching the file to the XMLHttpRequest object directly, as shown in the code:

xhr.file = file; // not necessary if you create scopes like this

This method is incorrect. Instead, the file should be wrapped into a FormData object, which constructs the proper multipart/form-data request payload:

var formData = new FormData();
formData.append("thefile", file);

Once the file is added to the FormData object, you can use xhr.send(formData); to submit the request. The file will be accessible on the server-side within $_FILES['thefile'] if using PHP.

Remember that you can consult resources like MDC and Mozilla Hack demos for further guidance on file uploads with Ajax XMLHttpRequest.