Nokia Application Installer is very picky with attributes in .jad and manifest file. I had similar problems when i started deploying applications to 6310i.
First of all make sure Jar-Size parameter in .jad has EXACT byte size of jar file. If that doesnt help download application developed for 6310i at midlet.org and copypaste the structure of .jad file there to your .jad file and edit it to your application profile, i think the parameters need to in correct order too.
Make sure you have same values in vendor and versions & other parameters which are at .jar & .jad files, that causes error
when you are trying to start application on 6310i.
If nothing of this didnt work look with jar & jad search
parameters here, there was some other usefull hints there
Sonera Juxto International
You need to configure your Web server to send the right Content-Type header: "text/vnd.sun.j2me.app-descriptor" for .jad files and "application/java-archive" for .jar files. If you're using Apache, create a file named ".htaccess" and include these lines in it:
AddType text/vnd.sun.j2me.app-descriptor jad
AddType application/java-archive jar
Put the .htaccess file in the directory containing your .jad and .jar or a parent directory.
I have the same problem, i download *.jad and *.jar file to my computer then i try to isntall by the nokia apllication installer via infra port to Nokia 5100.
The apllication installer say it's installed succesfully, but when i try to run from the phone i got the unknown file format error.
I download the Nokia 5100 emu, when i run the apllication from the computer it's work immaculately.
i am trying 45KB application download on Nokia 3100/6610i but during download and sometimes during installation, it says "Unknown Error" with a blank page. I have added mime type in my httpd.conf in apache server and also in tomcat web.xml which is connected to apache thru JK2 Connector. One more factor suspected is slow network connection which is present here.