Add Attachment.to_file to easily send an attachment.

The first thing someone will ask when someone sees this method is
"Why doesn't `send` just accept `Attachment`?". This question is fair
but it has an issue: exception propagation becomes confusing.

When we save a file and write it to memory an HTTP request is sent
similar to other API calls. Like all HTTP requests, these can fail.
Since these requests denote failure using HTTPException, if it were to
originate within `send` then it becomes confusing to know whether the
attachment saving itself failed or whether the sending failed.

For that reason, and to keep in-line with only 1 type of HTTP call per
method, it doesn't make sense for `send` to support `Attachment`.

discord/message.py

@@ -39,6 +39,7 @@ from .errors import InvalidArgument, ClientException, HTTPException
 from .embeds import Embed
 from .member import Member
 from .flags import MessageFlags
+from .file import File
 from .utils import escape_mentions
 
 
@@ -164,6 +165,32 @@ class Attachment:
         data = await self._http.get_from_cdn(url)
         return data
 
+    async def to_file(self):
+        """|coro|
+
+        Converts the attachment into a :class:`File` suitable for sending via
+        :meth:`abc.Messageable.send`.
+
+        .. versionadded:: 1.3.0
+
+        Raises
+        ------
+        HTTPException
+            Downloading the attachment failed.
+        Forbidden
+            You do not have permissions to access this attachment
+        NotFound
+            The attachment was deleted.
+
+        Returns
+        -------
+        :class:`File`
+            The attachment as a file suitable for sending.
+        """
+
+        data = await self.read()
+        return File(io.BytesIO(data), filename=self.filename)
+
 def flatten_handlers(cls):
     prefix = len('_handle_')
     cls._HANDLERS = {