If I use relative file path the plugin works but if I pass the same path as a parameter such as $MY_PACKAGE the plugin gives me an error:

```
io.jenkins.plugins.appcenter.AppCenterException: File not found: $MY_PACKAGE
```

I have a dynamic file name and unable to use a predefined value and thus my relative file path value is stored in a variable.

In the hockey app plugin the same path is recognized and used correctly, so when I migrated from hockey to appcenter, the build is broken and blocked.