Description:
In nm-connection-editor it's not possible to add a private key when configuring a wpa enterprise connection ( with TLS authentication ).
The GtkFileChooserDialog will not find files *.der *.pem *.p12 *.key, althought they are there with right permission, and it found them in previous versions.
When launching nm-connection-editor from terminal, the following output is printed while opening the GtkFileChooserDialog to select the private key:

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

(nm-connection-editor:2471): libnm-CRITICAL **: nm_utils_file_is_private_key: assertion 'out_encrypted == NULL || *out_encrypted == FALSE' failed

Additional info:
* package version(s)
networkmanager 1.2.0-3
nm-connection-editor 1.2.0-1
network-manager-applet 1.2.0-1
* config and/or log files etc.


Steps to reproduce:
1) Launch nm-connection editor and add a new wifi connection (or connect to a wpa enterpise network)
2) in the WiFi-Security tab select "Wpa & Wpa2 Enterprise" Security and then TLS as authentication
3) click on Private Key field