Thank you for the suggestion, this can be solved with a script. Simply but the code below in the script editor in a new script of type selection. This means that you first select the bytes that are part of the BASE64 string (in the hex editor), then run the script via the script menu.
~~~~~~~~~~~~~~~~~8<~~~~~~~~~~~~~~~~~
import base64
def processByteRange(byteView, byteArray, bytePos, byteLength):
# this method processes a sequence of bytes
"""parseByteRange method"""
data_string = byteView.readString(bytePos, byteLength, "ISO_8859-1")
data_bytes = data_string.encode('ascii')
decoded = base64.b64decode(data_bytes)
byteArray.deleteRange(bytePos, byteLength)
index = 0
for base64_char in decoded:
byteArray.insertByte(bytePos + index, base64_char)
index += 1
~~~~~~~~~~~~~~~~~8<~~~~~~~~~~~~~~~~~