Add default icons for FileAttachment annotations (bug 1230933)

*Please note:* This "borrows" the icons from Thunderbird.

According to the PDF specification, see https://web.archive.org/web/20220309040754if_/https://www.adobe.com/content/dam/acom/en/devnet/pdf/pdfs/PDF32000_2008.pdf#G11.2096626, we should be providing default icons for FileAttachment annotations without appearances.

test/pdfs/bug1230933.pdf.link

@@ -0,0 +1 @@
+https://bugzilla.mozilla.org/attachment.cgi?id=8696459

test/test_manifest.json

@@ -6064,6 +6064,14 @@
        "type": "eq",
        "annotations": true
     },
+    {  "id": "bug1230933",
+       "file": "pdfs/bug1230933.pdf",
+       "md5": "d8e7dce9ce79aa5a6121b21be8935429",
+       "link": true,
+       "rounds": 1,
+       "type": "eq",
+       "annotations": true
+    },
     {  "id": "issue14117",
        "file": "pdfs/issue14117.pdf",
        "md5": "9b1c33ad2f59f4e723c258e863149abf",