Filter files by extension

[Vasar007]

Describe the bug
There are no filter for upload files.

To Reproduce
Steps to reproduce the behavior:

1. Login to service;
2. Select two files with some extension;
3. Click "Send to analyze";
4. Files start uploading.

Expected behavior
You can easily add server filter by file extension and do not allow to upload not *.csv files.

Desktop (please complete the following information):

• Linux Ubuntu 18.04
• Mozilla Firefox 61.0.1

https://stackoverflow.com/questions/31804799/how-to-get-pdf-filename-with-python-requests