Jordy Meow

Rename the alternative text of your media easily directly using the Media Library, without a click.
BETA. This plugin was just created. I am waiting for the first feedbacks of my users to decide which way to go. The first thing I will implement is to update the ALT in the POSTS.
BE KIND. Again, this plugin is new. Please help us 🙂
More information available on http://apps.meow.fr/media-alt-renamer/.
<h4>Quickstart</h4>
<ol>
<li>Check the Media Library, in list view (not grid view).</li>
<li>Modify the Alt Text field, and simply type ENTER when it’s done.</li>
</ol>

Media Alt Renamer

Rename the alternative text of your media easily directly using the Media Library, without a click.

Kinski &amp; Bourke

This is a super simple plugin for uploaded file name sanitization.
Essentially, it’s to make your uploaded filenames easier for search engines to read, hence better for SEO, nicer to look at in the folder (who cares?), and should also help avoid any issues with naming conventions. 
You might be thinking that WordPress does a good enough job at renaming, but it still allows underscores and other symbols and characters that maybe don’t belong in a filename.
e.g. 
This: Æ, %20 æ Ï, Ö, Ü, Ÿ-boyz_%20__=++&, ©, §)+Ţ ţ++‰+ath-club-ÆØÅæøå–.pdf 
Becomes: AE-20-ae-I-O-U-Y-boyz-20-T-tath-club-AEOAaeoa.pdf 
This: —-___——]- hair space U+200A.jpg 
becomes: hair-space-U200A.jpg
The Original idea for this was done by Vaughn Royko and so we thought we would turn this into a simple plugin with a few options for those who are worried about messing with their functions file, and or can only install plugins and cant edit the Functions file.

SEO File Renamer

Very helpful and smooth plugin. Recommended!

Amazing plugin

A super simple plugin for uploaded file name sanitization.

Media Alt Renamer vs SEO File Renamer