[api-minor] Expose the /Desc-attribute of file attachments in the viewer (issue 18030)

In the viewer this will be displayed in the `title` of the hyperlink, which is probably the best we can do here given how the viewer is implemented.

test/pdfs/.gitignore

@@ -54,6 +54,7 @@
 !issue17056.pdf
 !issue17679.pdf
 !issue17679_2.pdf
+!issue18030.pdf
 !issue14953.pdf
 !issue15367.pdf
 !issue15372.pdf

test/unit/annotation_spec.js

@@ -4005,7 +4005,7 @@ describe("annotation", function () {
       const fileSpecRef = Ref.get(19, 0);
       const fileSpecDict = new Dict();
       fileSpecDict.set("Type", Name.get("Filespec"));
-      fileSpecDict.set("Desc", "");
+      fileSpecDict.set("Desc", "abc");
       fileSpecDict.set("EF", embeddedFileDict);
       fileSpecDict.set("UF", "Test.txt");
 
@@ -4035,6 +4035,7 @@ describe("annotation", function () {
       expect(data.annotationType).toEqual(AnnotationType.FILEATTACHMENT);
       expect(data.file.filename).toEqual("Test.txt");
       expect(data.file.content).toEqual(stringToBytes("Test attachment"));
+      expect(data.file.description).toEqual("abc");
     });
   });
 

test/unit/api_spec.js

@@ -1475,11 +1475,28 @@ describe("api", function () {
       const pdfDoc = await loadingTask.promise;
       const attachments = await pdfDoc.getAttachments();
 
-      const attachment = attachments["foo.txt"];
-      expect(attachment.filename).toEqual("foo.txt");
-      expect(attachment.content).toEqual(
+      const { filename, content, description } = attachments["foo.txt"];
+      expect(filename).toEqual("foo.txt");
+      expect(content).toEqual(
         new Uint8Array([98, 97, 114, 32, 98, 97, 122, 32, 10])
       );
+      expect(description).toEqual("");
+
+      await loadingTask.destroy();
+    });
+
+    it("gets attachments, with /Desc", async function () {
+      const loadingTask = getDocument(buildGetDocumentParams("issue18030.pdf"));
+      const pdfDoc = await loadingTask.promise;
+      const attachments = await pdfDoc.getAttachments();
+
+      const { filename, content, description } = attachments["empty.pdf"];
+      expect(filename).toEqual("Empty page.pdf");
+      expect(content instanceof Uint8Array).toEqual(true);
+      expect(content.length).toEqual(2357);
+      expect(description).toEqual(
+        "SHA512: 06bec56808f93846f1d41ff0be4e54079c1291b860378c801c0f35f1d127a8680923ff6de59bd5a9692f01f0d97ca4f26da178ed03635fa4813d86c58a6c981a"
+      );
 
       await loadingTask.destroy();
     });