files won't load in WP File Manager Free

Hi There!

We are planning on implementing a solution for our clients to access their docuents via web, and your plugin is being tested.

I downloaded and activated it in a staging site:

https://test.1980consultors.es/download/calendario-laboral-2019/

but when I try to insert the file into the document area in the backend I always get a red-cross warning over the file.

Can you tell me why?

Many thanks!

Pere