How to automatically open files downloaded from Safari?
You can add a file extension to the list of "safe" extensions. Once that is done, safari will launch whatever application is associated with that file type.

This is done by creating the file ~/Library/Preferences/com.apple.DownloadAssessment.plist containing the file extension you want opened.

Here is an example, which solves the problem for .torrent files:

&lt;!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com-PropertyList-1.0.dtd"&gt; 
&lt;plist version="1.0"&gt; 
&lt;dict&gt; 
 &lt;key&gt;LSRiskCategorySafe&lt;/key&gt; 
&lt;dict&gt; 
 &lt;key&gt;LSRiskCategoryExtensions&lt;/key&gt; 
&lt;array&gt; 
 &lt;string&gt;torrent&lt;/string&gt; 
 &lt;/array&gt; 
 &lt;/dict&gt; 
 &lt;/dict&gt; 
&lt;/plist&gt; 


More info here.

If the file exists already, edit it and add the &lt;string&gt;torrent&lt;/string&gt; line under the &lt;array&gt;tag