Change validation order for file extension

Currently, there are two validations on the backend exist for creating file extensions:
1. Duplicate file extensions
2. Invalid file extension

Now they are executed in the order specified above which may lead to the wrong results.

Steps to reproduce:
1. .csv file extension already exists on the server. Make POST request to data-import/fileExtensions endpoint with the following body

{
  "dataTypes": [],
  "description": "",
  "extension": "  .csv  ",
  "importBlocked": true
}

The problem is the extension field which has surrounding spaces.

Expected result:
The invalid file extension error returns from the server.

{
    "errors": [
        {
            "message": "fileExtension.extension.invalid",
            "parameters": []
        }
    ],
    "total_records": 1
}

Actual result:
The duplication error returns from the server.

{
    "errors": [
        {
            "message": "fileExtension.duplication.invalid",
            "parameters": []
        }
    ],
    "total_records": 1
}