Are you able to disable legacy markup for this form? The newer markup (used with forms created after version 2.5) presents these icons as trash bins instead – improving the user experience. Legacy markup can be toggled at Form → Settings → Form Options → Enable legacy markup.
You can hook into the filter gform_file_upload_markup with some custom code. There is an example on that page showing how to change delete button, though I don’t know if that specific example is relevant for legacy markup or the standard markup.