Yes, this is a limitation of the current Dropbox support. If you hit the red (!), the error message given will explain that. The limit is 150 MByte.
There is a complicated support for larger files which includes using some "upload session", but I didn't use that at first, as it would clearly not work in the background and it was a whole lot to implement.
We probably should revisit this, now that we positively know that we cannot ever support background upload anyway.
Let's add support for these larger foreground uploads
Per discussion on #196:
Let's add support for these larger foreground uploads