Add PAPERCLIP_ALLOWED_ATTACHMENT_TYPES env var to configure allowed
MIME types for issue attachments and asset uploads. Supports exact
types (application/pdf) and wildcard patterns (image/*, text/*).
Falls back to the existing image-only defaults when the env var is
unset, preserving backward compatibility.
- Extract shared module `attachment-types.ts` with `isAllowedContentType()`
and `matchesContentType()` (pure, testable)
- Update `routes/issues.ts` and `routes/assets.ts` to use shared module
- Add unit tests for parsing and wildcard matching
Closes#487
dec02225f1