As far as I understand, APEX 5.1 does not support Excel files to be loaded into tables.
I found this package that seems to make it possible to SELECT from Excel files, but it does not show how to use it with, for example, files loaded via the "File Browse" Item.
Now, I am very new to this environment, so please explain it from the beginning.
What I did is I upload the package script to the SQL workshop and executed it, without errors. But now?
APEX 5.1 doesn't support it out of the box, but you can use the EXCEL2COLLECTION plugin (available here).
It is very straightforward, just create a file browse page item with an upload button which calls an onsubmit process (e.g. CreateCollection) of type Excel2Collection[Plug In] - specify the file browse item, a collection name and the CSV separator, then you can do as you please with the data (e.g. you may want to run some validations on the data then insert it into a table where you can access it as normal).