bots: Add bot for uploading files to Zulip server.

Add file_uploader bot as an example of using
ExternalBotHandler's methods for uploading files
to Zulip server.

zulip_bots/zulip_bots/bots/file_uploader/doc.md

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+# File Uploader Bot
+
+This bot allows the user to upload a file with a given path to the Zulip server.
+
+## Usage
+
+Use this bot with any of the following commands:
+
+- `@uploader <local_file_path>` : Upload a file, where `<local_file_path>` is the path to the file
+- `@uploader help` : Display help message
+
+### Usage examples
+
+The following command will upload the file `/tmp/image.png` to the Zulip server:
+```
+@**uploader** /tmp/image.png
+```
+
+Here's an example response:
+
+> [image.png](https://server.zulipchat.com/user_uploads/3787/RgoZReSsfMjlQSzvVxjIgAQy/image.png)

zulip_bots/zulip_bots/bots/file_uploader/file_uploader.py

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+from typing import Any, Dict
+
+import os
+from pathlib import Path
+
+class FileUploaderHandler(object):
+    def usage(self) -> str:
+        return (
+            'This interactive bot is used to upload files (such as images) to the Zulip server:'
+            '\n- @uploader <local_file_path> : Upload a file, where <local_file_path> is the path to the file'
+            '\n- @uploader help : Display help message'
+        )
+
+    def handle_message(self, message: Dict[str, str], bot_handler: Any) -> None:
+        HELP_STR = (
+            'Use this bot with any of the following commands:'
+            '\n* `@uploader <local_file_path>` : Upload a file, where `<local_file_path>` is the path to the file'
+            '\n* `@uploader help` : Display help message'
+        )
+
+        content = message['content'].strip()
+        if content == 'help':
+            bot_handler.send_reply(message, HELP_STR)
+            return
+
+        path = Path(os.path.expanduser(content))
+        if not path.is_file():
+            bot_handler.send_reply(message, 'File `{}` not found'.format(content))
+            return
+
+        path = path.resolve()
+        upload = bot_handler.upload_file_from_path(str(path))
+        if upload['result'] != 'success':
+            msg = upload['msg']
+            bot_handler.send_reply(message, 'Failed to upload `{}` file: {}'.format(path, msg))
+            return
+
+        uploaded_file_reply = '[{}]({})'.format(path.name, upload['uri'])
+        bot_handler.send_reply(message, uploaded_file_reply)
+
+handler_class = FileUploaderHandler

zulip_bots/zulip_bots/bots/file_uploader/test_file_uploader.py

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+from unittest.mock import patch, MagicMock, Mock
+
+from zulip_bots.test_lib import (
+    get_bot_message_handler,
+    StubBotHandler,
+    BotTestCase,
+    DefaultTests,
+)
+
+from pathlib import Path
+
+class TestFileUploaderBot(BotTestCase, DefaultTests):
+    bot_name = "file_uploader"
+
+    @patch('pathlib.Path.is_file', return_value=False)
+    def test_file_not_found(self, is_file: Mock) -> None:
+        self.verify_reply('file.txt', 'File `file.txt` not found')
+
+    @patch('pathlib.Path.resolve', return_value=Path('/file.txt'))
+    @patch('pathlib.Path.is_file', return_value=True)
+    def test_file_upload_failed(self, is_file: Mock, resolve: Mock) -> None:
+        server_reply = dict(result='', msg='error')
+        with patch('zulip_bots.test_lib.StubBotHandler.upload_file_from_path',
+                   return_value=server_reply) as m:
+            self.verify_reply('file.txt', 'Failed to upload `/file.txt` file: error')
+
+    @patch('pathlib.Path.resolve', return_value=Path('/file.txt'))
+    @patch('pathlib.Path.is_file', return_value=True)
+    def test_file_upload_success(self, is_file: Mock, resolve: Mock) -> None:
+        server_reply = dict(result='success', uri='https://file/uri')
+        with patch('zulip_bots.test_lib.StubBotHandler.upload_file_from_path',
+                   return_value=server_reply) as m:
+            self.verify_reply('file.txt', '[file.txt](https://file/uri)')
+
+    def test_help(self):
+        self.verify_reply('help',
+                          ('Use this bot with any of the following commands:'
+                           '\n* `@uploader <local_file_path>` : Upload a file, where `<local_file_path>` is the path to the file'
+                           '\n* `@uploader help` : Display help message'))