Thanks for reporting this, exconny :)
'thickbox' class got hardcoded in 'simple' template by mistake, I was testing variable overriding (namely link, linkclass, and mimetype) within the template itself and forgot to clean up - sorry.
I'm going to leave mimetype='image' hardcoded in 'simple' template, because that template is meant to be used with images only. I've now made it so that if you set link to 'none', then the big picture on the left won't link to anything, but the thumbnails will still load the bigger picture.
And good job spotting the 'link_size' related linking bug when link is set to none, thanks :)
I've uploaded the version with these bugs fixed into repository as the current trunk version, you can download it from here: http://wordpress.org/extend/plugins/file-gallery/download/ (click on the Development Version link at the bottom) - it should be version 18.104.22.168-beta. If it's not that version, please try again in 10-15 minutes when the repository refreshes.
Let me know if that's all OK with you :)