What to do if certificate import fails with error ""Unable to open file"?

In some of the mobile devices the certificate import may fail if trying to do it by opening the certificate file with the File Manager application. The certificate import  fails even though the file contains a valid certificate. The following error message is displayed:

"Unable to open file"

The workaround is to place the certificate file in a web server and open it with the device web browser. Depending on the web server settings, the certificate file extension may have to be modified, e.g. with Microsoft IIS server the extension may need to be set to *.crt instead of *.cer.