If it is a large file, you may need to wait several seconds for the upload to complete. On Infictive, there is a limit of 2MB per file upload. This limit may be temporarily increased by user request.
After you've uploaded a file, you can use it in an article, with some special wiki syntax to create the link. See Help:Images.
Your file gets its very own 'Image Description Page' within the wiki (even if it isn't an image). You should edit this page to provide more information about the file. For example, many public wiki projects are quite sensitive to copyright issues, and so you should explain where you got the file from, and what its copyright status is, on the description page. Any other useful descriptive notes could also be added there.
There are a number of Special Pages to help you view and manage files which have been uploaded. Go to 'Special Pages' (in the toolbox on the left) and then see...
See Help:Special pages for details of the other information available here.