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/__tests__/attachment-types.test.ts

@@ -0,0 +1,89 @@
+import { describe, it, expect } from "vitest";
+import {
+  parseAllowedTypes,
+  matchesContentType,
+  DEFAULT_ALLOWED_TYPES,
+} from "../attachment-types.js";
+
+describe("parseAllowedTypes", () => {
+  it("returns default image types when input is undefined", () => {
+    expect(parseAllowedTypes(undefined)).toEqual([...DEFAULT_ALLOWED_TYPES]);
+  });
+
+  it("returns default image types when input is empty string", () => {
+    expect(parseAllowedTypes("")).toEqual([...DEFAULT_ALLOWED_TYPES]);
+  });
+
+  it("parses comma-separated types", () => {
+    expect(parseAllowedTypes("image/*,application/pdf")).toEqual([
+      "image/*",
+      "application/pdf",
+    ]);
+  });
+
+  it("trims whitespace", () => {
+    expect(parseAllowedTypes(" image/png , application/pdf ")).toEqual([
+      "image/png",
+      "application/pdf",
+    ]);
+  });
+
+  it("lowercases entries", () => {
+    expect(parseAllowedTypes("Application/PDF")).toEqual(["application/pdf"]);
+  });
+
+  it("filters empty segments", () => {
+    expect(parseAllowedTypes("image/png,,application/pdf,")).toEqual([
+      "image/png",
+      "application/pdf",
+    ]);
+  });
+});
+
+describe("matchesContentType", () => {
+  it("matches exact types", () => {
+    const patterns = ["application/pdf", "image/png"];
+    expect(matchesContentType("application/pdf", patterns)).toBe(true);
+    expect(matchesContentType("image/png", patterns)).toBe(true);
+    expect(matchesContentType("text/plain", patterns)).toBe(false);
+  });
+
+  it("matches /* wildcard patterns", () => {
+    const patterns = ["image/*"];
+    expect(matchesContentType("image/png", patterns)).toBe(true);
+    expect(matchesContentType("image/jpeg", patterns)).toBe(true);
+    expect(matchesContentType("image/svg+xml", patterns)).toBe(true);
+    expect(matchesContentType("application/pdf", patterns)).toBe(false);
+  });
+
+  it("matches .* wildcard patterns", () => {
+    const patterns = ["application/vnd.openxmlformats-officedocument.*"];
+    expect(
+      matchesContentType(
+        "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet",
+        patterns,
+      ),
+    ).toBe(true);
+    expect(
+      matchesContentType(
+        "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
+        patterns,
+      ),
+    ).toBe(true);
+    expect(matchesContentType("application/pdf", patterns)).toBe(false);
+  });
+
+  it("is case-insensitive", () => {
+    const patterns = ["application/pdf"];
+    expect(matchesContentType("APPLICATION/PDF", patterns)).toBe(true);
+    expect(matchesContentType("Application/Pdf", patterns)).toBe(true);
+  });
+
+  it("combines exact and wildcard patterns", () => {
+    const patterns = ["image/*", "application/pdf", "text/*"];
+    expect(matchesContentType("image/webp", patterns)).toBe(true);
+    expect(matchesContentType("application/pdf", patterns)).toBe(true);
+    expect(matchesContentType("text/csv", patterns)).toBe(true);
+    expect(matchesContentType("application/zip", patterns)).toBe(false);
+  });
+});