#Yes, Azure supports plain text files. In the documentation, they call it multi-line secret instead of a regular text file.
#I had a similar issue, I had to create a secret that contains a *.json file, which contains the private key among many other parameters.
#I solved the issue by using the following commands in a terminal.
#Create the secret
az keyvault secret set --name SECRET_NAME --vault-name KEYVAULT_NAME --file FILE_NAME_WITH_ABSOLUT_PATH
#Download the secret
az keyvault secret download --name SECRET_NAME --vault-name KEYVAULT_NAME --file FILE_NAME_WITH_ABSOLUT_PATH