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Whenever I attempt to upload a file to the Freemind Gallery, via the Upload file page, I get an Upload warning: "File extension does not match MIME type." and the upload fails. This occurs even if I tick the "Ignore any warnings" check box.
It looks like file uploads to the Gallery may have been broken for about three years now. The last *.mm file uploaded was on 1 January 2011 (Gita.mm at 22:40 on 1 Jan 2011) and a similar issue seems to have been raised before, on 14 March 2011: http://sourceforge.net/p/freemind/discussion/320014/thread/0a034ee7/
This issue only seems to affect .mm files. I was able to upload a .png file without problems.
The problem may be with the Freemind wiki configuration. Here is a comment about configuring MediaWiki to allow uploading of .mm files: http://www.mediawiki.org/wiki/Extension_talk:FreeMind#Need_to_patch_MimeMagic.php . The error message in this particular comment is different from the one I saw so the fix in the comment may not solve the problem with the Freemind Gallery. However, it may point the way to a fix.