This is a bug in the Save As dialog. If the filename is yourfile.FM, you
can choose MIF as the filetype in the Save as Type box, FrameMaker
changes the filename to yourfile.mif and saves as mif.
But, for some reason, if you have a filename of yourfile.MIF and choose
FM in the Save as Type dialog box, you get that error message and the
save doesn't happen. The workaround is to delete the MIF extension in
the filename. Then the save works fine and the file is renamed with an
FM extension.
In other words, saving FM to MIF works as expected. FrameMaker renames
the file extension and saves in the new format. But, in a MIF to FM
save, FrameMaker refused to rename the MIF extension. (Maybe there is a
reason for this?) You have to delete the extension or the save won't happen.
I'm going to report this as a bug.
Mike Wickham