Expecting one of "application/octet-stream", "text/plain; charset=dropbox-cors-hack".

The Dropbox API does not accept multipart requests, so you should not use MultipartEntityBuilder when constructing Dropbox API calls.

For information on what is expected for a call /2/files/upload, please refer to the documentation:

https://www.dropbox.com/developers/documentation/http/documentation#files-upload

As linked there, that endpoint is a "content-upload" style endpoint:

https://www.dropbox.com/developers/documentation/http/documentation#content-upload-endpoints 

The data for the file you're uploading should be sent in the request body, with a "Content-Type" header or "application/octet-stream" of "text/plain; charset=dropbox-cors-hack". (The first one is preferred.)

If you're using Java though, we recommend using the official Java SDK, which will do most of the work for you:

https://github.com/dropbox/dropbox-sdk-java