Download feature and URLInputFile (#332)

* Fix How to upload docs

* Rename BaseBot to Bot

* Add download_file method

* Add download method

* Add URLInputFile

* Add Downloadable to __init__ and __all__

* Fix ImportError for Python 3.7

* Related pages

* Improving docs

* Some speed

* staticmethod to classmethod

docs/api/upload_file.md

@@ -0,0 +1,97 @@
+# How to upload file?
+
+As says [official Telegram Bot API documentation](https://core.telegram.org/bots/api#sending-files) there are three ways to send files (photos, stickers, audio, media, etc.):
+
+If the file is already stored somewhere on the Telegram servers or file is available by the URL, you don't need to reupload it.
+But if you need to upload new file just use subclasses of [InputFile](./types/input_file.md). Here is available three different builtin types of input file:
+
+- `#!python3 FSInputFile` - [uploading from file system](#upload-from-file-system)
+- `#!python3 BufferedInputFile` - [uploading from buffer](#upload-from-buffer)
+- `#!python3 URLInputFile` - [uploading from URL](#upload-from-url)
+
+!!! warning "Be respectful with Telegram"
+    Instances of `InputFile` is reusable. That's mean you can create instance of InputFile and sent this file multiple times but Telegram is not recommend to do that and when you upload file once just save their `file_id` and use it in next times.
+
+## Upload from file system
+By first step you will need to import InputFile wrapper:
+```python3
+from aiogram.types import FSInputFile
+```
+
+Then you can use it:
+```python3
+cat = FSInputFile("cat.png")
+agenda = FSInputFile("my-document.pdf", filename="agenda-2019-11-19.pdf")
+```
+
+### FSInputFile(...)
+
+|Argument|Type|Description|
+|---|---|---|
+| path | `#!python3 Union[str, Path]` | File path |
+| filename | `#!python3 Optional[str]` | Custom filename to be presented to Telegram |
+| chunk_size | `#!python3 int` | File chunks size (Default: `64 kb`) |
+
+## Upload from buffer
+
+Files can be also passed from buffer (For example you generate image using [Pillow](https://pillow.readthedocs.io/en/stable/) and the want's to sent it to the Telegram):
+
+Import wrapper:
+
+```python3
+from aiogram.types import BufferedInputFile
+```
+
+And then you can use it:
+```python3
+text_file = BufferedInputFile(b"Hello, world!", filename="file.txt")
+```
+
+### BufferedInputFile(...)
+|Argument|Type|Description|
+|---|---|---|
+| `buffer` | `#!python3 bytes` | File path |
+| `filename` | `#!python3 str` | Custom filename to be presented to Telegram (Required) |
+| `chunk_size` | `#!python3 int` | File chunks size (Default: `64 kb`) |
+
+Also you can read buffer from file:
+
+```python3
+file = BufferedInputFile.from_file("file.txt")
+```
+
+### BufferedInputFile.from_file(...)
+|Argument|Type|Description|
+|---|---|---|
+| `path` | `#!python3 Union[str, Path]` | File path |
+| `filename` | `#!python3 Optional[str]` | Custom filename to be presented to Telegram |
+| `chunk_size` | `#!python3 int` | File chunks size (Default: `64 kb`) |
+
+## Upload from url
+
+If you need to upload a file from another server, but the direct link is bound to your server's IP, or you want to bypass native [upload limits](https://core.telegram.org/bots/api#sending-files) by URL, you can use [URLInputFile](#urlinputfile).
+
+Import wrapper:
+
+```python3
+from aiogram.types import URLInputFile
+```
+
+And then you can use it:
+```python3
+image = URLInputFile("https://www.python.org/static/community_logos/python-powered-h-140x182.png", filename="logo.png")
+```
+
+### URLInputFile(...)
+|Argument|Type|Description|
+|---|---|---|
+| `url` | `#!python3 str` | URL |
+| `filename` | `#!python3 Optional[str]` | Custom filename to be presented to Telegram |
+| `chunk_size` | `#!python3 int` | File chunks size (Default: `64 kb`) |
+| `timeout` | `#!python3 int` | Total timeout in seconds (Default: `30`) |
+
+## Related pages:
+
+- [Official documentation](https://core.telegram.org/bots/api#sending-files)
+- [aiogram.types.InputFile](types/input_file.md)
+- [How to download file?](download_file.md)