iOS apps can only read data in their own directory and thus only Safari would be able to use these files - so how could you create them and put them in Safari's directories?
From Apple's Filesystem basics development documentation
An iOS app’s interactions with the file system are limited mostly to
the directories inside the app’s sandbox. During installation of a new
app, the installer code creates a home directory for the app, places
the app in that directory, and creates several other key directories.
These directories constitute the app’s primary view of the file
system. Figure 1-1 shows a representation of the sandbox for an app.
Figure 1-1 Each iOS app operates within its own sandbox
Because it is in a sandbox, an app is generally prohibited from
accessing or creating files in directories outside of its home
directory. One exception to this rule occurs when an app uses public
system interfaces to access things such as the user’s contacts or
music. In those cases, the system frameworks handle any file-related
operations needed to read from or modify the appropriate data store
