Ensure `.ledger` format on import

[mstekiel]

I started using the app. Bumped into problem quickly when trying to create my ledger by importing a csv file. On import I saved it with name only, no extension, which caused synchronization problem on sync journal "Failed to sync, Exit status 1". It was not visible in the Ledger->Editor tab.
I had to take a look at the server to understand the issue, and realized that on import the new files were saved without .ledger extension. Changing it manually fixed the problem.
I think this could be easily fixed, for user convenience, by enforcing the .ledger format.

Environment
OS: Web app served from docker container with image: ananthakumaran/paisa:v0.7.4
Paisa Version: 0.7.4
App Variant: CLI

Bug
Saving an imported expenses list does not force the .ledger extension, thus if saving just with the name the ledger is not visible. Small bug, but easily fixed convenient for the users.

To Reproduce
Steps to reproduce the behavior:

1. Go to 'Ledger-import'
2. Load any format and click on save the ledger
3. The pop-up window shows hint expense.ledger
4. Save just with name, without extension

Expected behavior
Force the .ledger format by checking the desired file name. If it doesn's have the proper extension add it.