unified the filtering of the gallery items

ElementX/Sources/Services/Timeline/TimelineItems/Items/Messages/GalleryRoomTimelineItemContent.swift

@@ -15,6 +15,21 @@ nonisolated struct GalleryRoomTimelineItemContent: Hashable {
     /// The original textual representation of the formatted caption directly from the event (usually HTML code)
     var formattedCaptionHTMLString: String?
     let items: [GalleryItem]
+    
+    /// The attachments of the given types, paired with their position in the gallery so that they remain
+    /// identifiable once separated from it. Attachments of an unknown type are never included as there's
+    /// nothing to show for them.
+    /// - Parameter allowedTypes: The types to include, or `nil` to include them all.
+    func attachments(allowedTypes: [TimelineAllowedGalleryItemType]?) -> [(mediaIndex: Int, item: GalleryItem)] {
+        items.enumerated().compactMap { mediaIndex, item in
+            guard let allowedItemType = item.allowedItemType,
+                  allowedTypes?.contains(allowedItemType) ?? true else {
+                return nil
+            }
+            
+            return (mediaIndex, item)
+        }
+    }
 }
 
 /// Identifies a single attachment within a gallery message.