Nolan Chou

Sometimes we forgot change our file to a available name. So the plugin can help you auto change your upload file. 
I will add a filetype option of setting in next version. 
And a rename popup before the file upload if you change setting in future, default value is post_name.

wp_upload_rename

Change upload filename to random characters / random numbers / date / other by yourself.

ognjen.kuljic

Some very useful plugins like Regenerate Thumbnails leaves old files behind, so it is good practice to clean after it.
Features: 
*Remove all unused files from upload dir (Unattached files to posts or pages), 
*Remove all leftover files (forgotten files in upload dir).

qCleanup

This plugin allows you to delete unused and leftover files from upload dir. In one click you can rid of all unwanted files and reduce space usage.

wp_upload_rename vs qCleanup