We have confirmed there is an issue with the secondary upload process within the website for things like:
bulk download sending, product image upload, account logo image upload.
These functions rely on an older script that has stopped working in the last two days due to a configuration change we made. We are working on finding a way to fix it. Product file uploads are not being affected by this issue.
Update - This issue has finally been fixed. It was caused by a conflict in some dev we did a few days before the issue came up. We tried using a request.form call with a binary read and that caused a conflict. We adjusted the code and now it is fixed.