NetworkOnMainThreadException - connecting to Dropbox account

You're getting a NetworkOnMainThreadException, which means you're trying to make a network call on the main thread, which isn't allowed on Android. (The upload method makes a network call to the Dropbox API servers to send up the file content.) You should make this call on a background thread instead. This isn't specific to Dropbox, so there are several answers about how to do this on StackOverflow, e.g.:

https://stackoverflow.com/questions/6343166/android-os-networkonmainthreadexception

There's also an example of uploading in the background in the Android sample app included with the API v2 Java SDK:

https://github.com/dropbox/dropbox-sdk-java/blob/master/examples/android/src/main/java/com/dropbox/core/examples/android/UploadFileTask.java