Auto merge of #12156 - jimberlage:12153/mime-type-s3, r=aneeshusa

Adds explicit mime type to upload_nightly.sh

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This changes the script to pass the `--mime-type` option to `s3cmd`, as
described [here](http://s3tools.org/usage).  The MIME type used is
`application/octet-stream`.

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #12153 (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [X] These changes do not require tests because this is a minimal change to the upload_nightly.sh script.

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