Add the mimetype for smcbin file extension

[ricardoamador]

Changes were made to switch the file extension for AssetManifest.bin to smcbin which is not a normal file extension and therefore has no associated mimetype. This change can be found here: flutter/flutter#126077

Without this change the dashboard fails to display all icons in the top column above the builds.

With the added mimetype the icons are present.

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#127038

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• I listed at least one issue that this PR fixes in the description above.
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