Update user_faq.md: Adobe Illustrator files downloaded as PDF document
AI files produced by recent versions of Adobe Illustrator are at the same time valid PDF documents. Thus browsers will render them using their internal PDF preview mechanisms. Incertain cases, Firefox alters the file extension from *.ai to *.pdf when downloading attached AI files. This behaviour is unexpected from a user's point of view and may lead to the false assumption, that the downloaded file is not editable by Adobe Illustrator. Closes #11
Edited by Alexander Minges