If it works to Google drive and GMail it usually means that F-Stop does send correct data. What Dropbox app (I suppose you use Dropbox app for uploading) does with the data is out of F-Stop’s control. Maybe you should contract them and see why they can’t upload the image. Maybe they don’t support custom providers (which they should).
We had a few issues when WhatsApp was not handling data correctly and we created workarounds for that, but it’s not always possible to do it, as connection to some other app may break then.