Upload failing on iOS9.3 (Expecting a file upload)

Thanks for the additional information, it's very helpful.

The /1/files (POST) endpoint is an old upload method on the Core API, a.k.a. API v1. That endpoint is technically supported, and it looks like its behavior did change, which we're looking in to. However, we do strongly recommend using /1/files_put instead, as it's generally easier to implement and debug. Most apps use this endpoint instead.

Further, API v1, including both of these upload endpoints, supports both OAuth 1 and OAuth 2, with OAuth 2 being preferred.

The dbapi-2:// URL scheme on iOS isn't directly related to this. That's an undocumented URL scheme used by the official iOS Core SDK to handle the OAuth app authorization on iOS. We generally don't recommend using that manually, as it's not documented. If you do use it though, it will return a normal OAuth 1 access token to your app. You can also get OAuth 2 tokens from OAuth 1 tokens using /oauth2/token_from_oauth1.