feat: make attachment content types configurable via env var

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

server/src/attachment-types.ts

@@ -0,0 +1,67 @@
+/**
+ * Shared attachment content-type configuration.
+ *
+ * By default only image types are allowed.  Set the
+ * `PAPERCLIP_ALLOWED_ATTACHMENT_TYPES` environment variable to a
+ * comma-separated list of MIME types or wildcard patterns to expand the
+ * allowed set.
+ *
+ * Examples:
+ *   PAPERCLIP_ALLOWED_ATTACHMENT_TYPES=image/*,application/pdf
+ *   PAPERCLIP_ALLOWED_ATTACHMENT_TYPES=image/*,application/pdf,text/*
+ *
+ * Supported pattern syntax:
+ *   - Exact types:   "application/pdf"
+ *   - Wildcards:     "image/*"  or  "application/vnd.openxmlformats-officedocument.*"
+ */
+
+export const DEFAULT_ALLOWED_TYPES: readonly string[] = [
+  "image/png",
+  "image/jpeg",
+  "image/jpg",
+  "image/webp",
+  "image/gif",
+];
+
+/**
+ * Parse a comma-separated list of MIME type patterns into a normalised array.
+ * Returns the default image-only list when the input is empty or undefined.
+ */
+export function parseAllowedTypes(raw: string | undefined): string[] {
+  if (!raw) return [...DEFAULT_ALLOWED_TYPES];
+  const parsed = raw
+    .split(",")
+    .map((s) => s.trim().toLowerCase())
+    .filter((s) => s.length > 0);
+  return parsed.length > 0 ? parsed : [...DEFAULT_ALLOWED_TYPES];
+}
+
+/**
+ * Check whether `contentType` matches any entry in `allowedPatterns`.
+ *
+ * Supports exact matches ("application/pdf") and wildcard / prefix
+ * patterns ("image/*", "application/vnd.openxmlformats-officedocument.*").
+ */
+export function matchesContentType(contentType: string, allowedPatterns: string[]): boolean {
+  const ct = contentType.toLowerCase();
+  return allowedPatterns.some((pattern) => {
+    if (pattern.endsWith("/*") || pattern.endsWith(".*")) {
+      return ct.startsWith(pattern.slice(0, -1));
+    }
+    return ct === pattern;
+  });
+}
+
+// ---------- Module-level singletons read once at startup ----------
+
+const allowedPatterns: string[] = parseAllowedTypes(
+  process.env.PAPERCLIP_ALLOWED_ATTACHMENT_TYPES,
+);
+
+/** Convenience wrapper using the process-level allowed list. */
+export function isAllowedContentType(contentType: string): boolean {
+  return matchesContentType(contentType, allowedPatterns);
+}
+
+export const MAX_ATTACHMENT_BYTES =
+  Number(process.env.PAPERCLIP_ATTACHMENT_MAX_BYTES) || 10 * 1024 * 1024;