iPhone: Is it possible to rename the extension of a file? I have an iPhone 5. I am new to this world.  

Example file:

MyFile.zip.jpg


... must be renamed to:

MyFile.zip


... or, if not possible or feasible, this file :

MyFile.jpg


... must be renamed to:

MyFile.zip


Sumarizing: I would like to acquire some control of the extension (the characters after the last dot .) in the name of the files.

Methods easy to perform for unexperienced users are preferred.

What could this be used for?
Example:
Privacy is something of the past, we must admit it. But some people (like me) still would like to be able to send, say, a private email that only the receiver will read.  

Sending encrypted files is becoming forbidden for some email providers, like GMail (read their terms), so .zip or .rar encrypted are returned to the sender as failed. And installing cryptographic programs is a nightmare for common (and even experienced) users of something like an iPhone.
A simple workaround: send your files (a document if you just want to send text) inside an encrypted file (say .rar) like:

MyPrivateFile.rar


... but rename it to:

MyPrivateFile.jpg


The receiver must rename just the extension, and voila! : you both have succeeded in sending encrypted info via email.
But, as you know, the email program's interface in the iPhone is too simple. I am not very experienced in using it, but I think it does not allow... well, it does not allow many things, even file saving inside the phone, so renaming file extensions must be even more difficult.
Tho I have never had to do this before but I just checked and it seems that Dropbox allows you to rename files. So if you have a file attachment in email you can save an "image" file in an email to Dropbox and once there you can rename it.

A bit of a kludge but Dropbox accounts are free...