WhatsApp Tri-Crypt app for Google Android phones can be used as a background process to convert almost every version of WhatsApp backup archives, including .crypt7 files. What this will do is that it will make the original .crypt file from the .crypt7 file, which essentially represents crypt7 to crypt conversion.
Once you have the .crypt file you can use any of the available programs (for example WhatsApp Xtract) to decrypt and extract the contents of the encrypted archive.