I can't find "Attach a file or image" link mentioned in https://gitlab.com/wireshark/wireshark/-/wikis/SampleCaptures

edit

You first have to be a wiki editor, which I'm guessing you're not? Assuming that's the case, then visit the HowToEdit wiki page to get started. Once you have editing privileges, visit the Sample Captures wiki page (assuming that is the page where you want to attach the capture files to), and if you're signed in and have editing capabilities, you should be able to edit the page by clicking the Edit button on that page. From there, you should see the formatting bar mentioned.