gracefully handle .crdownload and other such temporary files

[Chris Danis]

The current design and implementation works fine if you're cping a file directly, but fails with many errors if you attempt using Chrome to download an image directly into a emojifs directory (for example).

We'd probably need to have some heuristics for recognizing a temporary file, cache their contents locally, and then do the upload to chat-backed storage in rename instead of in release.