How can I only sync files with a given extension?

Add a new sync item by clicking the + button in the toolbar.
In the General tab of the sync settings, type a title that describes the sync item. Then click on the first folder icon to select a source directory (in the image below, Documents), and on the second folder icon to select a destination (in the image below, Downloads/txt files).
In the Filter tab of the sync settings, enable the checkbox “Filter files matching any of the following expressions”. Then click on the + button below, select Filename in the popup menu, and double click the added expression to edit its value to “.txt”.
To turn the expression from excluded (the default) to included, open the menu by clicking on the arrow on the right or right-clicking the expression, and select Include.
Enable the checkbox “Filter following files”. Then click on the + button below and select the source directory that you chose in step 2. The source directory has now been added as an explicitly excluded file, out of which all the files matching the filter expression added earlier are included again. To learn more about explicitly filtered files and filter expressions, see Filters.
Click the red title bar button to close the sync settings.
Run the sync item by clicking on the Play button in the toolbar.