filename-regex.text

(?:^|\/|\\)(?:[a-zA-Z0-9\s\-\_])*\.[a-zA-Z]*$

This will check just the filename in a file upload and ignore the file path before the file name.
This will allow any filename with letters, numbers, dashes, underscores and spaces.

Use in jquery validator like this:

$.validator.addMethod("fileName", function(value, element) {
  return this.optional(element) || /(?:^|\/|\\)(?:[a-zA-Z0-9\s\-\_])*\.[a-zA-Z]*$/.test(value);
}, "File names can only contain letters, numbers, dashes, and underscores (and spaces).");

This post was helpful in determining how to ignore the file path before the file name: https://forum.jquery.com/topic/validate-a-filename-with-the-validation-pluging