Add "chemical" major MIME type to the image tables

The American Chemical Society suggested a new major MIME type for files
containing chemical data in 1998: http://dx.doi.org/10.1021/ci9803233
This suggestion got widely adopted and is now a de-facto-standard despite
not registered with IANA.

Applying this patch will allow us to continue with extension MolHandler
and PDBHandler.
http://fab.wmflabs.org/T352

- Fixes bug 66412 by creating a logic that will prevent running unneeded
  updates.

Bug: 66412
Change-Id: Ic45dc1bce796a0406ed8a84e6274df1c4bda4967