Accept per-file headers in multipart file POST messages.

- Each file in the `files` argument can now be a 4-tuple, instead of
  just a file, 2-tuple, or 3-tuple. If it is, the last value in the
  tuple is a dictionary of extra headers.

requests/models.py

@@ -17,6 +17,7 @@ from .structures import CaseInsensitiveDict
 
 from .auth import HTTPBasicAuth
 from .cookies import cookiejar_from_dict, get_cookie_header
+from .packages.urllib3.fields import RequestField
 from .packages.urllib3.filepost import encode_multipart_formdata
 from .packages.urllib3.util import parse_url
 from .exceptions import (
@@ -119,11 +120,14 @@ class RequestEncodingMixin(object):
         for (k, v) in files:
             # support for explicit filename
             ft = None
+            fh = None
             if isinstance(v, (tuple, list)):
                 if len(v) == 2:
                     fn, fp = v
-                else:
+                elif len(v) == 3:
                     fn, fp, ft = v
+                else:
+                    fn, fp, ft, fh = v
             else:
                 fn = guess_filename(v) or k
                 fp = v
@@ -132,11 +136,10 @@ class RequestEncodingMixin(object):
             if isinstance(fp, bytes):
                 fp = BytesIO(fp)
 
-            if ft:
-                new_v = (fn, fp.read(), ft)
-            else:
-                new_v = (fn, fp.read())
-            new_fields.append((k, new_v))
+            rf = RequestField(name=k, data=fp.read(), 
+                              filename=fn, headers=fh)
+            rf.make_multipart(content_type=ft)
+            new_fields.append(rf)
 
         body, content_type = encode_multipart_formdata(new_fields)