Issue with selecting non-image files during Share / Local

[aleksandrmelnikov]

When creating a task, you can select a folder of images.

• In a use-case, we had ~5000 images.
• Alongside, a hidden folder was included that had some text files
• The task fails to create with an error message of "Could not create the task on the server. KeyError: 'unknown'. Code: 400"
• Stack trace:

cvat          | 2019-09-18 21:18:19,909 DEBG 'rqworker_default_0' stderr output:
cvat          | 21:18:19 KeyError: 'unknown'
cvat          | Traceback (most recent call last):
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/worker.py", line 812, in perform_job
cvat          |     rv = job.perform()
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 588, in perform
cvat          |     self._result = self._execute()
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 594, in _execute
cvat          |     return self.func(*self.args, **self.kwargs)
cvat          |   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
cvat          |     return func(*args, **kwds)
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 244, in _create_thread
cvat          |     media = _validate_data(data)
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 175, in _validate_data
cvat          |     counter=counter,
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 169, in count_files
cvat          |     counter[mime].append(rel_path)
cvat          | KeyError: 'unknown'
cvat          | Traceback (most recent call last):
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/worker.py", line 812, in perform_job
cvat          |     rv = job.perform()
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 588, in perform
cvat          |     self._result = self._execute()
cvat          |   File "/usr/local/lib/python3.5/dist-packages/rq/job.py", line 594, in _execute
cvat          |     return self.func(*self.args, **self.kwargs)
cvat          |   File "/usr/lib/python3.5/contextlib.py", line 30, in inner
cvat          |     return func(*args, **kwds)
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 244, in _create_thread
cvat          |     media = _validate_data(data)
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 175, in _validate_data
cvat          |     counter=counter,
cvat          |   File "/home/django/cvat/apps/engine/task.py", line 169, in count_files
cvat          |     counter[mime].append(rel_path)
cvat          | KeyError: 'unknown'

CVAT Versions tested: 0.5.0, 0.4.0.x

Desired outcome:

• CVAT ignores any non-image / non-video files, and continues with task creation.
• It can be annoying when you're creating a task of 40k images and CVAT rejects it, so you have to find the non-image file.