Thanks for posting your workaround. Sorry to hear you ran into that problem! When it comes to uploading your file, either file type should be uploading the same amount of contacts. If you would like to email us your file, we could take a look at it and see if there is anything in the .CSV that would cause the system to think there are only 1,670 and not the full 6k.
Just reference this post and you can send your file to social_support(at)constantcontact(dot)com.
We would be happy to help!
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