thanks for your work!
I'm testing your plugin with infinitewp.
In the future, infinitewp will have premium add-ons to handle external sites transfers. In the meantime, and maybe as a good alternative, yours could very well do the job. Hope you find it interesting enough to investigate!
Feature request: in this scenario, an option to delete the successfully uploaded file could come handy. Or maybe an action/filter/constant, where we could hook or define: