The code is a mess, the limit semantics make no sense, and we have several other options that don't really fit non-images.
The best thing to do here is probably to just write a prop=fileinfo module from scratch so we don't have to worry about backwards compatibility, and then deprecate prop=imageinfo.
* Right now, iilimit specifies the max number of revisions to return per file, which is inconsistent with the rest of the API and isn't particularly sane. For fileinfo, filimits will limit the number of file-info-objects returned per result, and a separate "fioldversions" property (default 0, values integers or 'all') will specify the max number of revisions to be returned per file.
* fistart/fiend may result in the info for the current revision not being returned.
* iiprops has three different metadata properties. There really should be only one, and if possible it should be key-value pairs rather than a list of objects with key and value properties.
** Metadata needs to be separately continuable, see T86611.
* Figure out something sane to replace iiurlwidth/iiurlheight/iiurlparam. Maybe multi-valued fiparams?
* prop=stashimageinfo is very odd, it's a prop module but doesn't use any titles. It would make sense to me for prop=fileinfo to have a fifilekeys parameter instead of having a whole separate module for this.
* prop=videoinfo really isn't needed either. Instead we should make it possible for extensions to add additional info to the fileinfo response.