In the Menu Bar, click Edit > Preferences, then on the left side of the dialog box, select Import/Export > File Type > STL. After making any changes, press Apply to apply them and Save to save the changes. To exit and ignore any changes made, click the X.
Set the default exporting behavior for STL files.
STL Export Format - Specifies which format is used to write STL files:
Ascii - File is legible in the editor, but reading and writing is slower and requires more disk space.
Binary - File is illegible in the editor, but reading and writing is faster and uses less disk space.