Do not use integer index to check the response of the server

Created by: Quazgar

In tests/test_file.py in function test_thumbnails the response of the server is checked using an integer index. However, the index might change and the test fail without reason. Change the check to something reasonable.

Imported comments:

By Timm Fitschen on 2019-09-03T06:30:47.887Z

changed due date to December 04, 2019

By Timm Fitschen on 2019-08-30T06:19:31.932Z

changed due date to September 04, 2019

By Henrik tom Wörden on 2019-08-29T09:47:52.401Z

assigned to @timm.fitschen and unassigned @henrik_indiscale

By Henrik tom Wörden on 2019-08-15T12:01:47.165Z

mentioned in commit 2c986582

By Quazgar on 2019-08-13T09:48:04.446Z

assigned to @henrik_indiscale and unassigned @quazgar

By Quazgar on 2019-07-22T11:54:48.862Z

mentioned in commit 2c986582

By Henrik tom Woerden on 2019-07-22T11:54:48 (imported from GitLab project)

By Quazgar on 2019-07-22T11:54:17.556Z

changed due date to July 31, 2019

By Henrik tom Woerden on 2019-07-22T11:54:17 (imported from GitLab project)