[TYPO3] files with what suffix can be edited?

[Bing Du]

Hello,

Some files listed in File->Filelist can be edited (e.g. *.tmpl, *.html), 
but some don't (e.g. file.test).  Any documents on if you want a file to 
be editable, it has to have some specific suffix?

Thanks in advance,

Bing