Pull Requests Diffs for Objective-C extension files that include '+' don't work

In Objective-C its a common convention to create extension files for a class, that contain a '+' character in the middle of the filename, for example:
"NSError+MyProjectName.m"

When submitting a pull request with these files included, and browsing through the resulting diffs, I always get a message that the diff cannot be displayed:

#!obj-c

ProjectName/NSError+ProjectName.m
(i) While impressive, your diff is just too big!

"While impressive, your diff is just too big!"

These files are often very short, about 1-2k in size.