Saves the buffer into a file, returning #t if sucessful, #f otherwise.
The filename and format used to save the file can be retrieved with get-filename. In a wx:media-edit% instance, the format can be retrieved with get-file-format.
If filename is null, then the internally stored filename will be used; if filename is "" or if the internal name is unset or temporary, then the user will be prompted for a name. The possible values for format are listed at load-file. If show-errors? is #f, then dialogs reporting save errors are suppressed.