Files are stored without an extension

Related to #535

**Describe the bug**
When an NGO uploads something like their statute, they upload a PDF/docx file.
When trying to then download the file, that file is downloaded without an extension and we often have issues figuring out what kind of file it is.

**Steps to reproduce the behavior**
1. Register an NGO and upload a file
2. Go to the page of that NGO
3. Download the statute (it's a mandatory file)
4. The file name was changed and the extension was lost in the process

**Expected behavior**
4. The file has a different name but the extension is still kept there

**Screenshots**
Files downloaded from NGO Hub with no way of telling what kind of file it is: