New Post: files extensions

Hello! Thanks for this amazing solutions, Im new in SharePoint and this will help me a lot. Im wondering if there is a way to set files extentions, for example to deny a .php, .exe, etc file. And also I would like to know if is a way to limit the files to upload, I mean for example I just want that users could upload 20 files, if they select 21 there will appear a error/warning message or something like that.
Thanks in advance!