There's nothing in [[fileupload]] that suggests putting the file upload limit in the body of a page. That would indeed be a strange choice. Ikiwiki already uses [[PageSpecs|PageSpec]] in the Preferences page (for specifying locked pages, banned users, and subscriptions), and I had envisioned putting the file upload controls there, and possibly subsuming some of those other controls into them.
It's not clear to me that the concept of attaching files to a page fits ikiwiki very well; unlike most wikis, ikiwiki supports subdirectories and [[SubPages]], which allows for hierarchical placement of uploaded files, which is a much more flexible concept than simple attachment. Futhermore, the idea of listing all attached files at the bottom of a page seems somewhat inflexible. What if I want to make a podcast, using inline's existing support for that -- I won't want a list of every "attached" file at the bottom of my podcast's page then.
I don't understand why the file size would need to be stored in the index file; this information is available by statting the file, surely? Similarly, the mime type can be determined through inspection, unless there turns out to be a reason to need to cache it for speed.