I can't tell whether this is a bug with FireFox, with all other browsers, or with formidable.
Using formidable 1.0.2, node 0.4.8.

A file with a 3-byte character (tested with U+200F and U+2010) in its name, when uploaded via FireFox 5, is parsed as a field rather than a file. In my particular case, this results in "max field size exceeded" errors. Everything works correctly when using latest Chrome or IE8.