Are you sure the files are mp3's? Go to options to show hidden extensions. You may find they are not mp3's after all but something like ... xxxx.mp3.htm

Another possibility is the file name is too long & the file extension has been lost. In that case you'd need to manually add the.mp3 extension.

Also, go to iTunes preferences under Edit menu -> Advanced -> Importing & make sure the option to copy files to iTunes Music Library when importing is ticked.