Provide basename of file instead of full path

[selurvedu]

Also contains some minor changes in formatting and in the order of imports.

TODO before merging:

• Add unit tests for uploading file from disk (not just from a StringIO);
• Update robot_tests: filenames have to be compared with filenames instead of full paths.

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This change is

[coveralls]

Coverage decreased (-1.2%) to 72.189% when pulling c06c089 on selurvedu:basename into c97b052 on openprocurement:master.

[selurvedu]

PR for task 2: openprocurement/robot_tests#251

[coveralls]

Coverage decreased (-0.9%) to 71.893% when pulling 46daadb on selurvedu:basename into 4b61ace on openprocurement:master.