It says "Could not upload attachment to ./files/6991_81c2508bcf8672d0119eb96df82213ad."

I even can't upload the captured picture to this topic! How can I fix it?

May I suggest a work-around? I do this when it won't let me upload pictures. I upload the photo to any image hosting site, then use the image link from there to link the picture into the post. Hope that helps!

I haven't checked the rules on uploading images here but when this sort of thing happens it can be that the file is too big, image size too big, unsupported file type, file name too long, file name consists of unsupported characters.