(Import Multi-value Custom Data page) It is able to upload All type of files

Rep. Steps

1. 1. Login and navigate to "Import Multi-value Custom Data" page
   You can easy move to mentioned page entering to this link
   <site url>/civicrm/import/custom?reset=1
   In my cases, I was able to reproduce the issue on the following server http://dmaster.demo.civicrm.org/civicrm/import/custom?reset=1

1. 2. Pay attention on "Import Data File *" and you can see that it supports files only with comma-separated-values(CSV) format,
2. 3. Click on "Choose file" button
3. 4. It will open a window allowing any type of files

This is a wrong behavior, You shout not allow any wrong file upload to avoid future issues