DELL U2515H-display and Mac Mini (Late 2011) We're trying to get a DELL U2515H (2560 x 1440) running on a Mac Mini (late 2011). Currently it's connected with HDMI. 

Unfortunately it seems like resolutions only up to 1920 x 1200 are supported using HDMI on the mac mini:
HDMI port with support for up to 1920-by-1200 resolution
https://support.apple.com/kb/sp632?locale=en_US

However, resolutions up to 2560x1600 are supported using Thunderbolt:
Thunderbolt port with support for up to 2560-by-1600 resolution
https://support.apple.com/kb/sp632?locale=en_US

So the question is: 
which cable should we get to get to connect the Display -> Thunderbolt?
You should be able to use a Thunderbolt cable to attach the Mac Mini Thunderbolt port to the Dell Mini DisplayPort.