Great plugin, really like it!
I want (actually need) to allow uploading ANY file type. I'm surprised to see that WPCF7 forces at least one file type, which then means I have to list all possible extensions out there :)
Looking at the code in modules\file.php in wpcf7_file_validation_filter(), I notice that you escape the wildcard chars ?, *, +, . as well ... not only that, but it also seems to require the file to have an extension since you do a regex match on /\.(escaped_pattern)$/, whereas not all files actually have an extension.
All these restrictions are unnecessary and hinder functionality. I can easily hack your code but you should really permit your plugin users to be able to allow all file types. A simple . or .* as pattern would do.
Also, the safe way to escape strings for use in preg_*() functions is to use preg_quote() rather than using str_replace() only on ?, *, +, ., which leaves quite a big security hole (you might want to revise that function).
Otherwise, keep up the good work!
p.s. I admit I haven't read all the docs (i did read some), but a quick question: can I hook into the email function so I can do some custom stuff? A simple pointer would suffice.