When two plugins have their own file extensions (for example one with .h5p and one with .HTML) the upload form stops filtering by extension instead of just permit the valid extensions.

How to reproduce:

1. Install the provided contenttype_html plugin
2. Go to the content bank as admin and click on the "upload" button

Expected: the upload form should only permit upload .h5p and .HTML files

What happens: the extension filter is disabled.

Why this happens

The current upload form uses a content bank method called "get_supported_extensions_as_string" which returns a comma-separated extensions list. This works well with just one extension, the problem is when you have more than one extension. The file picker expects an array of extensions, not a comma-separated.

The solution is to delete the "get_supported_extensions_as_string" method and create anew method "get_supported_extensions" which is exactly the same but returning an array.