Add support for multiple file attachments.

This is a breaking change. No longer does Messageable.send have a
filename keyword argument, instead this is all handled through the
discord.File model. To upload many files you must specify a list
of discord.File objects.

discord/file.py

@@ -0,0 +1,73 @@
+# -*- coding: utf-8 -*-
+
+"""
+The MIT License (MIT)
+
+Copyright (c) 2015-2017 Rapptz
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+"""
+
+import os.path
+
+class File:
+    """A parameter object used for :meth:`abc.Messageable.send`
+    for sending file objects.
+
+    Attributes
+    -----------
+    fp: Union[str, BinaryIO]
+        A file-like object opened in binary mode and read mode
+        or a filename representing a file in the hard drive to
+        open.
+
+        .. note::
+
+            If the file-like object passed is opened via ``open`` then the
+            modes 'rb' should be used.
+
+            To pass binary data, consider usage of ``io.BytesIO``.
+
+    filename: Optional[str]
+        The filename to display when uploading to Discord.
+        If this is not given then it defaults to ``fp.name`` or if ``fp`` is
+        a string then the ``filename`` will default to the string given.
+    """
+
+    __slots__ = ('fp', 'filename', '_true_fp')
+
+    def __init__(self, fp, filename=None):
+        self.fp = fp
+        self._true_fp = None
+
+        if filename is None:
+            if isinstance(fp, str):
+                _, self.filename = os.path.split(fp)
+            else:
+                self.filename = getattr(fp, 'name', None)
+
+    def open_file(self):
+        fp = self.fp
+        if isinstance(fp, str):
+            self._true_fp = fp = open(fp, 'rb')
+        return fp
+
+    def close(self):
+        if self._true_fp:
+            self._true_fp.close()