Files get a .BAD extension

Ok I'm new to all this. I dragged the files I wanted (Invision Power Board) over using the local browser, to upload them to my site. The thing is some of the files go weird. For example, "lang_mod.php" turns into "lang_mod.php.bad" and it says the filetype is a "BAD file". Also the filetype should be 11kb but it changed to 10.7kb.

What's happened?

It doesn't do it for most of the files, and I've tried deleting the bad ones and uploading them again but the same thing happens.

Help greatly appreciated

File that get renamed to *.BAD failed the hash check against *.SFV / *.MD5SUM etc. files or the XCRC FTP server commands. Check if there any such hash files and if they are valid.