To modify what extensions the app will take:

First, go into concrete/config/base.php, and copy the line that looks like this:

(the exact extensions may be somewhat different)

define('UPLOAD_FILE_EXTENSIONS_ALLOWED', '*.flv;*.jpg;*.gif;*.jpeg;*.docx;*.xla;*.png;*.swf;*.doc;*.txt;
*.xls;*.csv;*.pdf;*.tiff;*.rtf;*.m4a;*.mov;*.wmv;*.mpeg;
*.mpg;*.wav;*.avi;*.mp4;*.mp3;*.qt;*.ppt;*.kml');

Paste or remove any extensions from this list for files you want to allow or disallow.

Then, paste this line into config/site.php.