By default, the Text Studio File Type recognizes the .txt file extension.
You can configure some aspects of SDL file types in Management > Linguistic Tool Setup > File Types.
Text (Regular Expression Delimited Text) Settings The Studio File Type Text is used for text files (.txt files) that are structured by patterns of embedded text. These patterns of text are specified in the form of regular expressions. For text files divided into columns, see the Character Delimited Text Studio File Type.