If you are represented by counsel, let him take care of it. Normally, the trustee will use a service called doclink. Some trustees use another system called Epiq. Our local chapter 13 trustee has created his own upload website.
In the old days (less than 4 years ago), we would fax or even mail hard copies of the taxes, paystubs, or any other documents, directly to the bk trustees. Now, we simply upload scanned documents.
If you are not represented by counsel, you still need to provide various documents to your bankruptcy trustee. Mailing a hard copy is safest, but if you call your trustee, he may allow you to email or fax those documents.