CompressUploads

This extension uses various techniques to reduce the size of uploaded files. Made for ULYSSIS VZW by Joachim Vandersmissen.

Features

Compress PDF files using qpdf.
Convert inefficient file formats (by default: BMP and TIFF) to PNG files.
Compress PNG files using configurable levels and filters.
Compress JPG files to configurable quality.
Automatically resize images exceeding a maximum width or height (or both).
Strip all EXIF data (orientation data will be processed, and ICC profiles are kept).

Install qpdf for PDF compression.
Install ImageMagick and Imagick for image processing.
Download the latest release, and put the CompressUploads folder in the extensions directory.
Add the following to LocalSettings.php:

wfLoadExtension( 'CompressUploads' );

As usual, configuration options can be added to LocalSettings.php using global variables.

Option	Value	Default Value	Description
`$wgCUCompressPdf`	`boolean`	`true`	Whether PDF files should be compressed using qpdf. This technique uses PDF Object Streams, which means the compression will have the most effect on PDF files containing a lot of indirect objects.
`$wgCUConvertImages`	`String[]`	`["image/bmp", "image/x-bmp", "image/x-ms-bmp", "image/tiff", "image/tiff-fx"]`	The MIME types of all inefficient image file formats that should be converted to PNG.
`$wgCUPngCompression`	`int`	`9`	The PNG compression level (0-9). This compression is lossless and should not cause any artifacts.
`$wgCUPngFilter`	`int`	`6`	The PNG compression filter. For advanced users.
`$wgCUJpgQuality`	`int`	`80`	The JPG compression quality (0-100). This compression is lossy: lower quality levels will cause compression artifacts.
`$wgCUMaxWidth`	`int`	`2000`	The maximum image width in pixels, larger images will be scaled accordingly. If not set, image width is unlimited.
`$wgCUMaxHeight`	`int`	`2000`	The maximum image height in pixels, larger images will be scaled accordingly. If not set, image height is unlimited.
`$wgCUStripExif`	`boolean`	`true`	Whether EXIF data should be stripped from uploaded images.

MediaWiki keeps temporary uploaded files in images/temp/. You could periodically delete these to free up space.
MediaWiki keeps deleted uploaded files in images/deleted/. You could delete these, provided you are absolutely sure you won't have to 'undelete' them.
The MediaWiki page Special:ListFiles shows every uploaded file on the wiki. These files can be sorted by size to quickly find out which files take up most space.
If EXIF data is stripped, metadata such as camera brand, aspect ratio, location is also removed. A win for privacy!