[pfefferle]

Split out of #3850.

Proposed changes:

• Remote_Posts::extract_attachments() is now public and takes a result limit and an optional type, so Display images attached to Fediverse comments #3850 can reuse it for comments. It inspects at most 20 candidates (or the limit, if higher).
• A single attachment or a bare URL is handled like a list, for attachment as well as for image.
• Attachment URLs must be absolute HTTP(S) URLs with a host. Relative URLs, javascript: and similar are skipped. Host checks (private IPs, ports) stay where they are, in the media cache before the download, so there is no DNS lookup while saving and a failed lookup can't drop an image.
• Attachments with a mediaType that is not image, audio or video (a PDF, for example) are now typed document and no longer added as an image. Before, they showed up as a broken image. Attachments without mediaType are treated like before: Audio and Video by their type, everything else as an image.
• Descriptions keep characters like < instead of having them stripped. They are escaped when the image is built.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Follow an account that posts images, audio or video and check that new posts show their media like before.
• Run npm run env-test -- --filter 'Test_Remote_Posts|Test_Functions_Media'.

Changelog entry

• Automatically create a changelog entry from the details below.

Changelog Entry Details

Significance

• Patch
• Minor
• Major

Type

• Added - for new features
• Changed - for changes in existing functionality
• Deprecated - for soon-to-be removed features
• Removed - for now removed features
• Fixed - for any bug fixes
• Security - in case of vulnerabilities

Message

Skip unusable attachments on posts from other servers instead of showing them as broken images.

The entry is already part of the branch.