nsopenpanel.m

NSOpenPanel *panel;
NSArray* fileTypes = [NSArray arrayWithObjects:@"pdf", @"PDF", nil];
panel = [NSOpenPanel openPanel];
[panel setFloatingPanel:YES];
[panel setCanChooseDirectories:NO];
[panel setCanChooseFiles:YES];
[panel setAllowsMultipleSelection:YES];
[panel setAllowedFileTypes:fileTypes];
int i = [panel runModal];
if (i == NSOKButton){
    return [panel URLs];
}

nsopenpanel.swift

let fileTypes = ["jpg", "png", "jpeg"]
let panel = NSOpenPanel()
panel.canChooseFiles = true
panel.canChooseDirectories = false
panel.allowsMultipleSelection = false
panel.allowedFileTypes = fileTypes
panel.beginSheetModal(for: window) { (result) in
    if result.rawValue == NSApplication.ModalResponse.OK.rawValue {
         // Do something with the result.
         let selectedFolder = panel.urls[0]
         print(selectedFolder)
    }
}