If the source app has a Share Menu, you can create an appex script that copies the file to pythonista.
i know several people have published their "save file to pythonista" app extension scripts.... here is mine:
#coding: utf-8
import urllib,urlparse
import appex,console,time
url=appex.get_url()
p=urlparse.urlparse(url)
f=urllib.unquote(urllib.unquote(urlparse.urlparse(appex.get_url()).path.split('/')[-1]))
urllib.urlretrieve(url,f)
import console
console.hud_alert(f)
appex.finish()
It turns out this works for urls (safari for example), but in other apps that expose files to the share menu, since it returns a file:// url to some app specific folder. for instance, if i click on a mail,attachment, and run pythonista script, get_url returns something like
file:///var/mobile/Library/Mail/IMAP-xxxxxx%40yahoo.com%40apple.imap.mail.yahoo.com/INBOX.imapmbox/Attachments/yyyyyy/2/file.ext
I suspect in python3 you'd replace urlretrieve with just an urlopen, read() and write(), I admit I am lazy and enjoy the convienece or urlretrieve. Also, this probably would be a little more robust if it checked for an existing file before clobbering it, and perhaps auto incrementing and appending an integer until the file does not exist.