Add function to check valid extension for Backgound image URL.

[neeraj3029]

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• [x] bug report
• [ ] feature request
• [ ] support request => Please do not submit support requests here, instead ask your query in our Gitter channel

Current behavior: Any form of text is accepted

Expected behavior: Only valid image URL should be accepted.

Screenshots of the issue:

Your operating system MacOS Mojave

Would you like to work on the issue? Yes