Snapshot resource handler: Use attachmentReaderType

isSnapshotAttachment() checks the link mode (needs to be IMPORTED_URL),
but we actually allow viewing all attachments with text/html MIME types
as "snapshots" in the reader. Don't throw on any HTML attachment that
the reader is willing to open.

chrome/content/zotero/ZoteroProtocolHandler.jsm

@@ -107,7 +107,7 @@ function ZoteroProtocolHandler() {
 						.slice(params.groupID !== undefined ? 4 : 3)
 						.filter(Boolean);
 					if (resourcePathParts.length) {
-						if (!item.isSnapshotAttachment()) {
+						if (item.attachmentReaderType !== 'snapshot') {
 							return this._errorChannel(`Item for ${uriPath} is not a snapshot attachment -- cannot access resources`);
 						}