feat(uploads): Restrict attachment type

[jjbayer]

DESCRIBE YOUR PR

The upload endpoint can only be used for plain attachments, so we should restrict the attachment_type item header.

ref: https://linear.app/getsentry/project/super-sized-attachments-d6ccf1b64c8d

IS YOUR CHANGE URGENT?

Help us prioritize incoming PRs by letting us know when the change needs to go live.

• Urgent deadline (GA date, etc.):
• Other deadline:
• None: Not urgent, can wait up to 1 week+

SLA

• Teamwork makes the dream work, so please add a reviewer to your PRs.
• Please give the docs team up to 1 week to review your PR unless you've added an urgent due date to it.
  Thanks in advance for your help!

PRE-MERGE CHECKLIST

Make sure you've checked the following before merging your changes:

• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
develop-docs	Ready	Preview, Comment	Apr 2, 2026 8:33am

1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
sentry-docs	Ignored	Preview	Apr 2, 2026 8:33am

[sentry]

Bug: The documentation for attachment placeholders is contradictory. The attachment_type field is marked as REQUIRED in one place but is described as conditional and omitted from an example elsewhere.
_{Severity: MEDIUM}

Suggested Fix

Update the documentation to be consistent. The prose on line 259 should be changed to state that attachment_type is always required. The JSON example on line 391 should be updated to include the "attachment_type":"event.attachment" field to match the specification.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent.
Verify if this is a real issue. If it is, propose a fix; if not, explain why it's not
valid.

Location: develop-docs/sdk/telemetry/attachments.mdx#L254

Potential issue: The documentation for attachment placeholders in
`develop-docs/sdk/telemetry/attachments.mdx` contains conflicting information regarding
the `attachment_type` field. A newly added table on line 254 specifies that
`attachment_type` is a `REQUIRED` field. However, pre-existing prose on line 259 implies
it is conditional, stating it should be included "if it was set". Furthermore, an
example envelope on line 391 omits the `attachment_type` field entirely. This
contradictory guidance will likely lead to inconsistent SDK implementations, with some
treating the field as required, some as conditional, and others as optional, undermining
protocol compliance.

_{Did we get this right? 👍 / 👎 to inform future reviews.}