first topic split file name

The first topic in my output is picking up its name from the .fm file rather than from the heading. Which option controls that?

AttachmentSize
mif2htm.ini_.txt2.72 KB

First file name cannot be altered

That's always been how it worked; the name of the first file cannot be altered, because it is opened before the ini is read. However, you can set [HTMLOptions]StartingSplit=Yes, and specify the format of the first head as a split point. Then you are left with an empty file which need not be included in the deliverables. See par. 34.8.3 ,"Using custom markers to name output files".