Fix requests comments upload files s3

[Samk13]

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Description

AI use disclosure: Most of this PR was generated with Codex, with additional guidance, cleanup, and adjustments to match the existing codebase style.

• fix attached file insertion in RichEditor
• detect file type from the selected upload, not dialog metadata
• insert attached uploads directly into the editor as images
• partially closes: Requests: Corrupted File Downloads from Requests Conversation Uploads invenio-app-rdm#3417

Note that I only tested these changes locally. I have not yet verified them with an
S3 file origin that differs from the localhost app origin yet.

2026-04-11_14-54-51.mov

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