In a project which I’m working on, the team make an intensive use of text file to include with SSI, and these files are named conventionally  .inc, Aptana by default opens these custom extensions with the default system’s editor (notepad on windows). We can tell Aptana to open the .inc or whatever extension we want with the Aptana’s text editor by simply doing the following: Choose window/preferences/Aptana/Editors/Generic Text, click add and then type our desired extension (in my case *.inc). Now if we double click over a .inc file in the project tree view, it will be opened inside Aptana.
ps. The path to the settings panel may change according to the version of Aptana/Eclipse installed (I’m using the Aptana plugin for Eclipse 3.4.1)