I'm a new guy on windows phone environment, and I wonder why I can't install another app out from marketplace. And I also try to develop an app for windows phone after finished I put the xap file from bin to the Onedrive then download that file, but the message said that the windows phone didn't support this file type. Do you have any idea why it happens? My phone os version is 7.5. Sorry for the silly questions.
You cant install an app like that. You can only do so using the deployment tool that comes with the developer kit. For that your phone must be developer unlocked. Click the link for more information about it: http://www.addictivetips.com/mobile/install-xap-files-on-your-windows-phone-7-device-how-to-guide/ (Recommended)
Also if you like to install a downloaded xap file, please check the following link for information: http://pocketnow.com/windows-phone/easily-install-windows-phone-7-homebrew-apps-with-xap-installer and http://thewp7phones.com/install-xap-files-on-windows-phone-7/
Althought the above two processes are not recommended!