The only possible option is to interrogate the Pega application directly including the attachment button. Then create the automation based on the business use case. 

for attaching a whole file , it is tricky , 

if it is a json / xml / text file, or any file that can be sent as a stream reader , or string, then , pass the whole file string to pega , and then try to manipulate it .

if its a pdf / other specific format , then Follow above comment by @rushabh

this approach is followed in the reverse process as well ( sending data from Pega to robotics )