I asked the following question on the TeX forum and was told to ask on an Apple forum instead:
I often use Mac OS's Quick Look (pressing the spacebar while a file is highlighted in Finder) to preview documents. Before updating to MacTex 2015, this worked fine on my .tex documents, displaying the plain text. After the update it just shows a blank icon. How can I fix this?
Further information: this seems to have only stopped working on one login, but still works correctly on others.