Update FileMessage Attribute name to match 

## Problem
what `__repr__` show
![Screenshot 2020-03-25 at 11 19 43 PM](https://user-images.githubusercontent.com/8089231/77559759-224e0900-6eef-11ea-8ffc-37360c2c4da6.png)

what I expect
```python
event.message["fileName"] # or
event.message.fileName
```

but this is what to code
```python
event.message.file_name
```

for `fileSize` as well