feat: add HEIC/HEIF image format support

[grantfitzsimmons]

Fixes #43

This updates the base image to Ubuntu 26.04, Python to 3.14, and add libheif-plugin-libde265 dependency for HEIC/HEIF image processing.

Summary by CodeRabbit

• New Features

  • Added support for generating thumbnails from HEIC and HEIF images.
  • Expanded thumbnail handling to cover more image/document formats.

• Bug Fixes

  • Updated thumbnail processing so PDFs, TIFFs, HEIC, and HEIF files are consistently converted to PNG previews.
  • Improved environment compatibility by using a newer base system and Python version.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: bd592072-680d-4855-8d15-a130e4b893d5

📥 Commits

Reviewing files that changed from the base of the PR and between 21d1170 and 1d3ac50.

📒 Files selected for processing (3)

• Dockerfile
• server.py
• settings.py

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📝 Walkthrough

Walkthrough

Adds HEIC/HEIF MIME types to the CAN_THUMBNAIL allowlist and to the PNG-thumbnail-generation branch in resolve_file(). Updates the Dockerfile to use Ubuntu 26.04 and Python 3.14, and adds the libheif-plugin-libde265 package for HEIF decoding support.

Changes

HEIC/HEIF Thumbnail Support

Layer / File(s)	Summary
Thumbnail mimetype allowlist and PNG generation settings.py, server.py	CAN_THUMBNAIL now includes image/heic and image/heif, and resolve_file()'s PNG-thumbnail branch handles these mimetypes alongside PDF/TIFF, with an updated comment.
Docker base image and HEIF decoder dependency Dockerfile	Base image bumped to ubuntu:26.04, Python upgraded from 3.12 to 3.14 (including the venv bootstrap step), and libheif-plugin-libde265 added to installed apt packages.

Possibly related issues

• Support HEIC/HEIF thumbnails specify7#8246: Describes HEIC/HEIF thumbnail support requirements, which are directly implemented by the MIME allowlist and HEIF decoder changes in this PR.

🚥 Pre-merge checks | ✅ 4 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Automatic Tests	⚠️ Warning	The PR changes thumbnail-selection logic but adds no automated tests; the repo also has no test files/config and the workflow only wraps dockerization.	Add automated coverage for settings.CAN_THUMBNAIL and server.resolve_file() covering HEIC/HEIF plus the existing PDF/TIFF thumbnail paths.
Testing Instructions	⚠️ Warning	The PR description has no testing section, and the docs don’t give HEIC/HEIF-specific verification steps.	Add explicit steps to rebuild the image and verify .heic/.heif thumbnail generation and download paths.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: adding HEIC/HEIF image format support.
Linked Issues check	✅ Passed	The PR adds thumbnail support for .heic and related HEIF MIME types, satisfying issue #43.
Out of Scope Changes check	✅ Passed	The Docker and runtime updates are described in the PR objectives as part of HEIC/HEIF support.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-43

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