diff --git a/src/calibre/ebooks/oeb/polish/download.py b/src/calibre/ebooks/oeb/polish/download.py
new file mode 100644
index 0000000000..ea4d6b6995
--- /dev/null
+++ b/src/calibre/ebooks/oeb/polish/download.py
@@ -0,0 +1,122 @@
+#!/usr/bin/env python2
+# vim:fileencoding=utf-8
+# License: GPLv3 Copyright: 2016, Kovid Goyal <kovid at kovidgoyal.net>
+
+from __future__ import (unicode_literals, division, absolute_import,
+                        print_function)
+import shutil, os, posixpath, cgi, mimetypes
+from collections import defaultdict
+from contextlib import closing
+from urlparse import urlparse
+from multiprocessing.dummy import Pool
+from functools import partial
+from tempfile import NamedTemporaryFile
+from urllib2 import urlopen
+
+from calibre import as_unicode, sanitize_file_name2
+from calibre.ebooks.oeb.polish.utils import guess_type
+from calibre.ebooks.oeb.base import OEB_DOCS, iterlinks, barename, OEB_STYLES
+from calibre.ptempfile import TemporaryDirectory
+from calibre.web import get_download_filename_from_response
+
+
+def is_external(url):
+    try:
+        purl = urlparse(url)
+    except Exception:
+        return False
+    return purl.scheme in ('http', 'https', 'file', 'ftp')
+
+
+def iterhtmllinks(container, name):
+    for el, attr, link, pos in iterlinks(container.parsed(name)):
+        tag = barename(el.tag).lower()
+        if tag != 'a' and is_external(link):
+            yield el, attr, link
+
+
+def get_external_resources(container):
+    ans = defaultdict(list)
+    for name, media_type in container.mime_map.iteritems():
+        if container.has_name(name) and container.exists(name):
+            if media_type in OEB_DOCS:
+                for el, attr, link in iterhtmllinks(container, name):
+                    ans[link].append(name)
+            elif media_type in OEB_STYLES:
+                for link in container.iterlinks(name):
+                    ans[link].append(name)
+    return ans
+
+def get_filename(original_url_parsed, response):
+    ans = get_download_filename_from_response(response) or posixpath.basename(original_url_parsed.path) or 'unknown'
+    ct = response.info().get('Content-Type', '')
+    if ct:
+        ct = cgi.parse_header(ct)[0].lower()
+        if ct:
+            mt = guess_type(ans)
+            if mt != ct:
+                exts = mimetypes.guess_all_extensions(ct)
+                if exts:
+                    ans += exts[0]
+    return ans
+
+def download_one(tdir, timeout, url):
+    try:
+        purl = urlparse(url)
+        with NamedTemporaryFile(dir=tdir, delete=False) as dest:
+            if purl.scheme == 'file':
+                src = lopen(purl.path, 'rb')
+                filename = os.path.basename(src)
+            else:
+                src = urlopen(url, timeout=timeout)
+                filename = get_filename(purl, src)
+            with src:
+                shutil.copyfileobj(src, dest)
+            filename = sanitize_file_name2(filename)
+            mt = guess_type(filename)
+            if mt in OEB_DOCS:
+                raise ValueError('The external resource {} looks like a HTML document ({})'.format(url, filename))
+            if not mt or mt == 'application/octet-stream' or '.' not in filename:
+                raise ValueError('The external resource {} is not of a known type'.format(url))
+            return True, (url, sanitize_file_name2(filename), dest.name, mt)
+    except Exception as err:
+        return False, url, as_unicode(err)
+
+
+def download_external_resoures(container, urls, timeout=60):
+    failures = {}
+    replacements = {}
+    with TemporaryDirectory('editor_download') as tdir:
+        pool = Pool(10)
+        with closing(pool):
+            for ok, result in pool.imap_unordered(partial(download_one, tdir, timeout), urls):
+                if ok:
+                    url, suggested_filename, downloaded_file, mt = result
+                    with lopen(downloaded_file, 'rb') as src:
+                        name = container.add_file(suggested_filename, src, mt, modify_name_if_needed=True)
+                    replacements[url] = name
+                else:
+                    url, err = result
+                    failures[url] = err
+    return replacements, failures
+
+def replacer(url_map):
+    def replace(url):
+        r = url_map.get(url)
+        replace.replaced |= r is not None
+        return url if r is None else r
+    replace.replaced = False
+    return replace
+
+def replace_resources(container, urls, replacements):
+    url_maps = defaultdict(dict)
+    changed = False
+    for url, name in urls.iteritems():
+        replacement = replacements.get(url)
+        if replacement is not None:
+            url_maps[name][url] = container.name_to_href(replacement, name)
+    for name, url_map in url_maps.iteritems():
+        r = replacer(url_map)
+        container.replace_links(name, r)
+        changed |= r.replaced
+    return changed