Ultimate Member uses an upload library (jquery.uploadfile.js), which adds an extra, empty field to the request for every upload. The field has no name and contains the text undefined:
The bug occurs in the library’s serializeData(): dynamicFormData returns an empty object {} by default, which is serialized to an empty text string, and the result is a field with an empty name and the value undefined. This happens on every upload, regardless of browser and file type.
