update mode for drop_upload

[karthik]

Need to investigate the use of drop_upload(..., mode = "update") when one has the use case of updating the file but preserving the version history. I am not entirely certain if mode = "overwrite") destroys the version history entirely. This would be an enhancement and not a deal breaker for the impending v2 update.

[ClaytonJY]

Went ahead and chucked it into 0.9; feel free to move to 0.8 if you get ambitious.