export file naming with correct extensions

src/main/java/com/sparrowwallet/sparrow/control/FileWalletExportPane.java

@@ -44,6 +44,7 @@ public class FileWalletExportPane extends TitledDescriptionPane {
 
         FileChooser fileChooser = new FileChooser();
         fileChooser.setTitle("Export " + exporter.getWalletModel().toDisplayString() + " File");
+        fileChooser.setInitialFileName(wallet.getName() + "-" + exporter.getWalletModel().toDisplayString().toLowerCase() + "." + exporter.getExportFileExtension());
 
         File file = fileChooser.showSaveDialog(window);
         if(file != null) {

src/main/java/com/sparrowwallet/sparrow/io/ColdcardMultisig.java

@@ -71,6 +71,11 @@ public class ColdcardMultisig implements WalletImport, KeystoreFileImport, Walle
         return "Import file created by using the Settings > Multisig Wallets > Export XPUB feature on your Coldcard";
     }
 
+    @Override
+    public String getExportFileExtension() {
+        return "txt";
+    }
+
     @Override
     public Wallet importWallet(InputStream inputStream, String password) throws ImportException {
         Wallet wallet = new Wallet();

src/main/java/com/sparrowwallet/sparrow/io/Electrum.java

@@ -184,6 +184,11 @@ public class Electrum implements KeystoreFileImport, WalletImport, WalletExport
         return "Import an Electrum wallet";
     }
 
+    @Override
+    public String getExportFileExtension() {
+        return "json";
+    }
+
     @Override
     public void exportWallet(Wallet wallet, OutputStream outputStream) throws ExportException {
         try {

src/main/java/com/sparrowwallet/sparrow/io/WalletExport.java

@@ -7,4 +7,5 @@ import java.io.OutputStream;
 public interface WalletExport extends Export {
     void exportWallet(Wallet wallet, OutputStream outputStream) throws ExportException;
     String getWalletExportDescription();
+    String getExportFileExtension();
 }