Diagnostics > Edit File contains a file editor that allows editing and creating files on the pfSense filesystem.
Enter the filename to edit or create in Save / Load from path, or click Browse and locate the file. Once the path is filled in, press Load. If a new file is being created, type the path and filename to which it will be saved.
Edit or create the text, then press Save when finished. For existing files, the contents will be saved. For new files, the file will be created.
NOTE: Be careful when choosing a file to edit! It is very easy to edit the wrong file, or break a piece of code, and render the system unusable. Use of this tool is not recommended except under guidance of support or when there is sufficient knowledge to use it without causing unintended side effects.