kubernetes deployment mounts secret as a folder instead of a file

Secrets vs ConfigMaps

Secrets let you store and manage sensitive information (e.g. passwords, private keys) and ConfigMaps are used for non-sensitive configuration data.
As you can see in the Secrets and ConfigMaps documentation:

A Secret is an object that contains a small amount of sensitive data such as a password, a token, or a key.

A ConfigMap allows you to decouple environment-specific configuration from your container images, so that your applications are easily portable.

Mounting Secret as a file

It is possible to create Secret and pass it as a file or multiple files to Pods.
I've create simple example for you to illustrate how it works.Below you can see sample Secret manifest file and Deployment that uses this Secret:
NOTE: I used subPath with Secrets and it works as expected.

---apiVersion: v1kind: Secretmetadata:  name: my-secretdata:  secret.file1: |    c2VjcmV0RmlsZTEK  secret.file2: |    c2VjcmV0RmlsZTIK---apiVersion: apps/v1kind: Deploymentmetadata:...    spec:      containers:      - image: nginx        name: nginx        volumeMounts:        - name: secrets-files          mountPath: "/mnt/secret.file1"  # "secret.file1" file will be created in "/mnt" directory          subPath: secret.file1        - name: secrets-files          mountPath: "/mnt/secret.file2"  # "secret.file2" file will be created in "/mnt" directory          subPath: secret.file2      volumes:        - name: secrets-files          secret:            secretName: my-secret # name of the Secret            

Note: Secret should be created before Deployment.

After creating Secret and Deployment, we can see how it works:

$ kubectl get secret,deploy,podNAME                         TYPE                                  DATA   AGEsecret/my-secret             Opaque                                2      76sNAME                    READY   UP-TO-DATE   AVAILABLE   AGEdeployment.apps/nginx   1/1     1            1           76sNAME                         READY   STATUS    RESTARTS   AGEpod/nginx-7c67965687-ph7b8   1/1     Running   0          76s$ kubectl exec nginx-7c67965687-ph7b8 -- ls /mntsecret.file1secret.file2$ kubectl exec nginx-7c67965687-ph7b8 -- cat /mnt/secret.file1secretFile1$ kubectl exec nginx-7c67965687-ph7b8 -- cat /mnt/secret.file2secretFile2

Projected Volume

I think a better way to achieve your goal is to use projected volume.

A projected volume maps several existing volume sources into the same directory.

In the Projected Volume documentation you can find detailed explanation but additionally I created an example that might help you understand how it works.Using projected volume I mounted secret.file1, secret.file2 from Secret and config.file1 from ConfigMap as files into the Pod.

---apiVersion: v1kind: Secretmetadata:  name: my-secretdata:  secret.file1: |    c2VjcmV0RmlsZTEK  secret.file2: |    c2VjcmV0RmlsZTIK---apiVersion: v1kind: ConfigMapmetadata:  name: my-configdata:  config.file1: |    configFile1  ---apiVersion: v1kind: Podmetadata:  name: nginxspec:  containers:  - name: nginx    image: nginx    volumeMounts:    - name: all-in-one      mountPath: "/config-volume"      readOnly: true  volumes:  - name: all-in-one    projected:      sources:      - secret:          name: my-secret          items:            - key: secret.file1              path: secret-dir1/secret.file1            - key: secret.file2              path: secret-dir2/secret.file2      - configMap:          name: my-config          items:            - key: config.file1              path: config-dir1/config.file1

We can check how it works:

$ kubectl exec nginx -- ls /config-volumeconfig-dir1secret-dir1secret-dir2    $ kubectl exec nginx -- cat /config-volume/config-dir1/config.file1configFile1$ kubectl exec nginx -- cat /config-volume/secret-dir1/secret.file1secretFile1$ kubectl exec nginx -- cat /config-volume/secret-dir2/secret.file2secretFile2

If this response doesn't answer your question, please provide more details about your Secret and what exactly you want to achieve.