Support image file readering

The more we know about your system and use case, the more easily and likely we can help.

#### Environment info
* OS: Windows 10
* App Version (`Command/Ctrl + ,`): v1.5.1-alpha-1

#### Description of the problem / feature request / question:
essentially, gist is a repo, user can add any file to the repo. include image file, like .jpg ,png etc. Hope Lepton support image file, even upload image file

#### If possible, provide a sample gist or screenshot:


#### If possible, provide the log files
- Mac: `~/Library/Application Support/Lepton/logs`
- Windows: `C:\Users\\AppData\Roaming\Lepton\logs`
- Linux: `~/.cached/Lepton/logs`
>The [debug mode](https://github.com/hackjutsu/Lepton/wiki) is recommended when producing the logs.
>You can send your log file to cosmo.lepton@gmail.com if don't want to post it here.