With HTML 5 the HTML Input File tag can have a multiple attribute and allow multiple files to be selected. But you only get that in an application if the application supports it. FileMaker and Xojo don't not really support it, but we can add it with our plugin:
For MBS FileMaker Plugin you can use the WebView.InstallMultiFileOpenDialog function. The next time you click such a file button, the dialog allows multiple selection if website supports it also.
With MBS Xojo Plugin, you can use WebUIDelegateMBS class. We'll add there new methods and parameters for multiple files support.
All coming soon with new plugin prerelease.