Fix unknown formats in tf detection api export

[zhiltsov-max]

Summary

Related #35

• Fixed unknown image formats on TF Detection API export - they should not lead to errors and produce an empty image format field
• Supported jpeg along with jpg

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT