Switch to dc_msg_save_file() for exporting attachments

[iequidoo]

IIUC currently the app implements copying of a file on its own. Also when two files with the same are sent, the second one gets the name <filename>-1.<ext> and if it's exported then, this mangled name is used. It doesn't look very bad, but i'd suggest to use the original file name, at least when no such file yet exists in the target dir.