From Xojo Documentation
The file extension for the file type, or several extensions separated by semicolons, e.g., "jpg;jpeg". A period before the file type is required on Windows, but is optional on other platforms.
The following example dynamically creates two FileTypes, one for JPEG files and one for PNG files. You must provide a value for the Extensions properties. It then displays an Open File dialog box that allows the selection of only those two file types.