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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[ptamarit]

Do images always have an alt provided? I'm wondering if there is not a better way to distinguish between images and files.

[Samk13]

The reason for this change is that meta.filetype was not reliable in this flow. TinyMCE was reporting images as file, so image uploads were not inserted correctly.

If I understand correctly, alt is not used to distinguish images from files; it is only added after the file has already been classified as an image.

If you have a better suggestion, please let me know and I can update the fix.

[ptamarit]

I cannot think of another way the documented way using meta.filetype.
If this doesn't work with S3, I guess this is the best we can do.