Is there a difference of meaning between "edit" and "modify" in this context?

To me modify is more extensive than edit.

You could do both by changing the contents, but if you simply changed the properties then you might be said to be be modifying but not editing.

If this is an instruction in some software then you could say edit is changing the contents while modify might also mean rename or delete the file - but really there is no difference.

In almost every single case, "edit" and "modify" have the same meaning, and are used for the same purpose. However, there is a difference when it comes to this:

Edit page content give your users the ability to change the content in a page
Modify page give your users the ability to publish items

That's only a very very tiny slight difference, and I don't think it really counts. I'd say they are identical.

There are ways to modify something other than to edit. Every edit is a modification, but not every modification is an edit. Specifically a "modification" is any change. An "edit" is correction, revision, or preparation for publication.

To give an example of the difference in your specific example, many files are produced as a result of an automated process. The result of the automated process should not be edited. Instead, to modify the file, the inputs to the automated process should be edited and the automated process repeat to create a new output file. This is an example of a file that may be modified but should not be edited.

Here's a real world example from my sendmail.cf file:

DO NOT EDIT THIS FILE! Only edit the source .mc file.

The file may be modified, of course, by editing the .mc file and rebuilding the .cf file.