View content from uploaded file in editor

I have a custom metabox that let’s the user upload .csv files while creating a page.
The content of the .csv is shown on the created page at the “front-end”.

I would like to show the content on the admin page in the WYSIWYG-editor so the user can change if necessary.
Can anybody tell me how to view the content of a uploaded file?

I don’t really think the code to the metabox and upload function is necessary but if anybody wants is let me know.

M.