Pull Request description

The original logic could potentially lead to the case when the prepared resource will be deleted using "revokeObjectURL" before it is imported. This situation can be stably reproduced in case "js-function.model" is copied to widget extensions.
In the new logic, the resource will be removed only after import is finished.

General checklist

• You have reviewed the guidelines document.
• Labels that classify your pull request have been added.
• The milestone is specified and corresponds to fix version.
• Description references specific issue.
• Description contains human-readable scope of changes.
• Description contains brief notes about what needs to be added to the documentation.
• No merge conflicts, commented blocks of code, code formatting issues.
• Changes are backward compatible or upgrade script is provided.
• Similar PR is opened for PE version to simplify merge. Crosslinks between PRs added. Required for internal contributors only.

Front-End feature checklist

• Screenshots with affected component(s) are added. The best option is to provide 2 screens: before and after changes;
• If you change the widget or other API, ensure it is backward-compatible or upgrade script is present.
• Ensure new API is documented here