The following are reserved keywords (any combination of upper/lower case) and should not be used as a part of filenames that are affiliated in any way with web files or image files.
angelfire
chdir
killop
mech
For instance, mech-engr.html would get a 406 Not Acceptable Apache error. (Even though the web author meant innocently "mechanical engineer")
Note that even if the above groups of letters are a subset (even without lexical parsing) of a filename (ex., mechanicals.jpg), that triggers the 406 filter.
We apologize for any inconvenience that this may cause. Rename the filename to avoid the above exact sequence of characters.