Cannot Connect with TLS Client Certificate

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## What did you do?

1. I created a Postgres SSL server certificate, client public key and private key, each in .pem format.
2. I created a new Postico connection, setting "Use TLS Client Certificate". I specify my SSL server certificate .pem file.
3. I try to connect and receive an error: `could not load private key file ".pem": no start line`

## What did you expect to happen?

I expected to see a successful connection, as I can successfully connect via the command line:

```
psql "sslmode=verify-full sslrootcert=.pem \
sslcert= sslkey= \
hostaddr= \
host= \
port=5432 \
user=postgres dbname="
```

## What actually happened?

Cannot connect.

## What software versions are you using?

Postico version:
1.3.1 (2153)

macOS version:
10.13.2

PostgreSQL version:
9.6.6

I suspect this is because I need to specify also an SSL cert and key, right?

No source file, test, or entry point is identified. Start by reproducing the connection with the provided psql parameters and Postico 1.3.1 on macOS 10.13.2, then compare how the server certificate, client certificate, and private key are selected. Done means the TLS client-certificate connection succeeds with the separate certificate and key files.

Written by the indexing model from the issue text.