Report the correct mime type for exported data

Technically text/plain is correct-ish, but text/csv is more correct.

app/src/main/java/protect/card_locker/ImportExportActivity.java

@@ -340,7 +340,7 @@ public class ImportExportActivity extends AppCompatActivity
                     Uri outputUri = FileProvider.getUriForFile(ImportExportActivity.this, BuildConfig.APPLICATION_ID, path);
                     Intent sendIntent = new Intent(Intent.ACTION_SEND);
                     sendIntent.putExtra(Intent.EXTRA_STREAM, outputUri);
-                    sendIntent.setType("text/plain");
+                    sendIntent.setType("text/csv");
 
                     // set flag to give temporary permission to external app to use the FileProvider
                     sendIntent.setFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);