Ref: Remove Attachment ContentType since the Server infers it

[marandaneto]

📜 Description

Ref: Remove Attachment ContentType since the Server infers it

💡 Motivation and Context

Closes #1589

💚 How did you test it?

📝 Checklist

• I reviewed the submitted code
• I added tests to verify the changes
• I updated the docs if needed
• No breaking changes

🔮 Next steps

[github-actions]

	Fails
🚫	Please consider adding a changelog entry for the next release.

Instructions and example for changelog

Please add an entry to CHANGELOG.md to the "Unreleased" section. Make sure the entry includes this PR's number.

Example:

## Unreleased

- Remove Attachment ContentType since the Server infers it ([#1874](https://github.com/getsentry/sentry-java/pull/1874))

If none of the above apply, you can opt out of this check by adding #skip-changelog to the PR description.

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