fix file picking from dropbox

[gtselos]

flutter/flutter#21547

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[Hixie]

Hard-coding knowledge of dropbox seems like the wrong approach here. What if there is some other similar service? Should every Flutter app hard-code knowledge about every such service?

[Hixie]

Since the CLA hasn't been signed, we won't be able to accept this PR and I'm going to close it. Please don't hesitate to reopen this PR or submit a new one if you have the opportunity to sign the CLA. Thanks!