Add ContentType to headers sent to AWS S3

[arzafran]

ContentType header was missing in next-tinacms-s3 package, making AWS serve SVGs as application/octet-stream.

This PR solves it by pulling the mimetype from the file and if it's not available it fallbacks to application/octet-stream

[changeset-bot]

🦋 Changeset detected

Latest commit: 00f7853

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
next-tinacms-s3	Minor

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