General Actions:
A page can contain attachments which can then later be referenced from within a page. For example this is very useful for attaching images and displaying them in the page's content.
There are 3 possibilities to attach a file:
You can add an attachment using the Add menu:

When clicked, it'll direct you to the Attachments tab located at the bottom of the page:

View the page on which you wish to attach a file and then click on the "Attachments" tab at the bottom of the page.

Note that the file selector that appears allows you to select as many files as you wish to upload. Thus you don't need to add attachments one by one.
Edit the page on which you wish to attach a file using the WYSIWYG editor and click on "Link > Attached File..." in the toolbar. You can also click on "Image > Attached Image" and you'll get a similar popup window as shown below:
Note that you can create a link to the attachment/image directly in that same popup window.
If you're using the Wiki editor, you can use the Wiki Syntax to display an image inline or link to an attachment.
If you're using the WYSIWYG editor, then use the "Link > Attached File..." or "Image > Attached Image" buttons as shown above.
If your file is on a remote server and you wish to link to it from your wiki, simply use the URL of the link as the content of your wiki link. For example, using the wiki editor you'd write:
For displaying an image located on a remote server, use the following HTML code snippet:
See the XWiki Syntax Guide for more details.
The maximum size of an attachment is limited by the administrator. By default it is set at about 32 megabytes.
For more information about how XWiki attachments work and how to configure/tune the attachment store, you can examine the administrator's guide to attachments.