Check the forum settings in your profile. There is an option about the upload method. I think the default method causes some people problems.
Edit:
Under Preferences, uncheck the "Use Flash when uploading" option.
No problem. I got bit by this beforeAnd all this years I have been posting presets and images using dropbox because I was afraid to ask....
Than you for the tip!
To celebrate it, and to verify if it works, now I am posting this preset ready to play David Gilmour's 5AM.
(Use a strat with EMG SA - SPC at 10, EXG at 2)