Support FileAttachments with hash-signs in the filename (issue 15729)

The reason for the issue is that we use the generic `getFilenameFromUrl` helper function, which was originally intended for regular URLs.
For the filenames we're dealing with in FileAttachments, we really only want to strip the path when one exists[1].

---
[1] See [bug 1230933](https://bugzilla.mozilla.org/show_bug.cgi?id=1230933) for an example of such a case.

src/display/annotation_layer.js

@@ -2496,7 +2496,7 @@ class FileAttachmentAnnotationElement extends AnnotationElement {
     super(parameters, { isRenderable: true });
 
     const { filename, content } = this.data.file;
-    this.filename = getFilenameFromUrl(filename);
+    this.filename = getFilenameFromUrl(filename, /* onlyStripPath = */ true);
     this.content = content;
 
     this.linkService.eventBus?.dispatch("fileattachmentannotation", {

src/display/display_utils.js

@@ -334,15 +334,16 @@ function isPdfFile(filename) {
 /**
  * Gets the filename from a given URL.
  * @param {string} url
+ * @param {boolean} [onlyStripPath]
  * @returns {string}
  */
-function getFilenameFromUrl(url) {
-  const anchor = url.indexOf("#");
-  const query = url.indexOf("?");
-  const end = Math.min(
-    anchor > 0 ? anchor : url.length,
-    query > 0 ? query : url.length
-  );
+function getFilenameFromUrl(url, onlyStripPath = false) {
+  let end = url.length;
+  if (!onlyStripPath) {
+    const anchor = url.indexOf("#");
+    const query = url.indexOf("?");
+    end = Math.min(anchor > 0 ? anchor : end, query > 0 ? query : end);
+  }
   return url.substring(url.lastIndexOf("/", end) + 1, end);
 }
 

test/unit/display_utils_spec.js

@@ -190,6 +190,13 @@ describe("display_utils", function () {
       const url = "https://server.org/filename.pdf?foo=bar";
       expect(getFilenameFromUrl(url)).toEqual("filename.pdf");
     });
+
+    it("should get the filename from a relative URL, keeping the anchor", function () {
+      const url = "../../part1#part2.pdf";
+      expect(getFilenameFromUrl(url, /* onlyStripPath = */ true)).toEqual(
+        "part1#part2.pdf"
+      );
+    });
   });
 
   describe("getPdfFilenameFromUrl", function () {

web/pdf_attachment_viewer.js

@@ -118,7 +118,10 @@ class PDFAttachmentViewer extends BaseTreeViewer {
     for (const name of names) {
       const item = attachments[name];
       const content = item.content,
-        filename = getFilenameFromUrl(item.filename);
+        filename = getFilenameFromUrl(
+          item.filename,
+          /* onlyStripPath = */ true
+        );
 
       const div = document.createElement("div");
       div.className = "treeItem";