fix: preserve dropped image attachments

[shriram-svg]

Fixes #2847.

What changed

• Detect dropped image files by extension as well as MIME type, so SVG files with missing/non-standard MIME types are still inserted as images.
• Only run EXIF/canvas rotation for JPEGs. SVG, PNG, GIF, and other non-JPEG image files are copied as-is, preserving their original bytes and extension.
• Generate new :storage/... attachment markdown with POSIX separators so Windows drops do not insert backslashes into markdown image URLs.
• Added a regression test for dropped SVG/PNG files.

Verification

• npx --yes -p node@20 node node_modules/jest/bin/jest.js tests/dataApi/attachmentManagement.test.js --runInBand --testURL=http://localhost
• npx --yes -p node@20 node node_modules/eslint/bin/eslint.js browser/main/lib/dataApi/attachmentManagement.js tests/dataApi/attachmentManagement.test.js
• git diff --check

Notes:

• yarn install --frozen-lockfile is blocked by the legacy lockfile's git://github.com/adobe-webplatform/eve.git#eef80ed dependency, which Yarn v1 cannot resolve from advertised refs. I used npm install --package-lock=false --ignore-scripts --legacy-peer-deps for local verification without committing dependency metadata.
• Full npm run jest -- --runInBand --testURL=http://localhost still has existing unrelated storage lookup failures in createNote, deleteFolder, createNoteFromUrl, createFolder, and find-storage; the updated attachmentManagement suite passes.
• Full npm run lint -- --quiet still fails on existing project-wide styleName/prettier errors unrelated to these files; ESLint passes on the touched files.