Does Apple accept apps which has a simple HTTPServer or FTPServer for transferring files via wi-fi?
Yes, I do believe you can include some or all of that functionality. Look at GoodReader's features, for example. It does wifi transfers, and it will connect to network shares. Some supported protocols include FTP, SFTP, SMB, AFP, etc. IIRC, I used a copy of it on my iPhone and a copy on my iPad to transfer files.

Good luck!