Prevent incompatible items from being pasted on compose screen

Tusker/Screens/Compose/ComposeAttachmentsViewController.swift

@@ -108,6 +108,10 @@ class ComposeAttachmentsViewController: UITableViewController {
     }
 
     override func canPaste(_ itemProviders: [NSItemProvider]) -> Bool {
+        guard itemProviders.allSatisfy({ $0.canLoadObject(ofClass: CompositionAttachment.self) }) else {
+            return false
+        }
+        
         switch mastodonController.instance.instanceType {
         case .pleroma:
             return true