[PsyCrow-code]

Summary

Fixes #12934.

Some mail senders label PDF attachments as application/octet-stream, even though their filenames end in .pdf or .PDF. Nextcloud Mail then displays a generic attachment icon and does not offer the PDF viewer.

This change enables previews for these specific attachments without changing their declared MIME type or normal download behavior.

Changes

• Use the files_pdfviewer handler for application/octet-stream attachments with a case-insensitive .pdf filename extension.
• Display the PDF icon in both message view and message-list attachment tags, including results already stored in the attachment metadata cache.
• When a preview is requested, check the %PDF- signature server-side. Reject invalid candidates with HTTP 415 and show a native Nextcloud error notification instead of opening the viewer.
• Preserve the original MIME type, regular attachment download URL, and downloads of other file types.
• Add frontend and backend regression coverage for valid and invalid PDF candidates, explicit non-PDF MIME types, and cached metadata.

Performance and limitations

• Attachment bodies are not fetched to determine icons or while displaying the mailbox.
• Server-side signature checking happens only on a preview request.
• A manual Chrome test of an 87,917-byte PDF showed a single network transfer, followed by a viewer request served from the browser disk cache. HTTP cache reuse is not guaranteed in every browser or configuration.
• The %PDF- check confirms only the file signature, not that the complete file is a structurally valid or safe PDF. Parsing and rendering remain the viewer's responsibility.
• Preview still requires files_pdfviewer; normal downloads are unaffected.

Testing

• Eight Vitest tests passed against current upstream/main (6.0.0-alpha.1).
• ESLint passed for both changed frontend files.
• Production Webpack build succeeded against current upstream/main.
• php -l passed for the four changed PHP source/test files.
• git diff --check upstream/main...HEAD passed.
• Manually tested on Nextcloud 35.0.2 with Mail 5.12.6: valid PDF preview, invalid PDF rejection (HTTP 415), nonblocking native notification, normal downloads, icons in message view and message list, and cached attachment metadata.
• PHP unit tests require a suitable Nextcloud core test environment / CI.

The separately developed local multiline message-list customization is not included.

AI assistance

• This contribution was partly developed with assistance from ChatGPT and manually reviewed and functionally tested.

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