Content of Email Diff is not readable (BB-13068)

For one repositroy i activated the Hook "Email Diff", but the Content of the Email looks like this

"MSBuZXcgY29tbWl0IGluIENhdGVyaW5nUG9ydGFsLUZpbGVhZG1pbjoKCmh0dHBzOi8vYml0YnVj
a2V0Lm9yZy9ub3Jkc29ubmUvY2F0ZXJpbmdwb3J0YWwtZmlsZWFkbWluL2NvbW1pdHMvNGJlM2Jj
NDYwND0KOTYvCkNoYW5nZXNldDogICA0YmUzYmM0NjA0OTYKQnJhbmNoOiAgICAgIG1hc3RlcgpV
..."

It's a repository with HTMl/CSS and some images.