Solución a este problema a la hora de subir un fichero de una imagen con extensión .png
You try to upload a picture using the JCE editor in Joomla, and you get a red error message: Error 403: invalid mime type. This was really frustrating at first, but it’s one of the quickest fixes ever. A simple setting in you JCE configuration will make it go away. Here’s how.
Step 1: Find the JCE Profile Manager
In your Joomla Administration:
Go to Components > JCE Editor > Profiles
Go to the Editor Profiles tab.
Choose the profile you want to edit by clicking on the name of the profile. In this case we only need to change the Default profile. There are other profiles but they may not be active. Check the status column for the green circle with the check mark. That indicates the active profiles.
On the Editor Parameers tab page you will see a section labled Filesystem. Click on Filesystem.
When you get to the Filesystem parameters scroll down to find “Validate Mime Type”
Change this to No.
Be sure to save your change.
Now try uploading a picture. The error message doesn’t appear and the picture gets uploaded