Image pre- or post-processing

[grinapo]

Description:
Did a brief test on the images hosted o nmy homeserver.

• Using jpegoptim on jpegs generally saved 40-60% of the original size.
• Converting images to better formats (my test was about WebP but it's just one) resulted 10-20% of the original size (and actually reducing the quality resulted more than 90% savings).
  Images are large, and it's getting bigger every minute. Optimising, mostly without quality loss would be a great saver.

[deltabeard]

I would also like a post-processing capability added to Synapse. Even something as simple as calling a script or application with the file path of the image uploaded to the server given as a parameter would be good.

I'm having a problem where screenshots are being uploaded in lossless PNG format which are very large in size when said images contain many colours (eg. screenshots of gameplay). By adding post-processing, it would allow me to compress these images appropriately.