(dev/core#1044) Extension/MIME matching should be case insensitive

(dev/core#1044) Extension/MIME matching should be case insensitive

Overview
--------

For CIVI-SA-2019-15, the delivery of file attachments was tightened to
ensure that the file-extension and mime-type were in agreement.  However,
the check yields a false-negative in the common case where the filename has
been capitalized.  It should treat `foo.jpg`, `foo.JPG`, and `FOO.JPG` as
equally valid.

Before
------

* When viewing a contact profile image ending with `.JPG`, there is an error
  message, `Supplied mime-type does not match file extension`.

After
-----

* When viewing a contact profile image ending with `.JPG`, the image is
  delivered.

Comments
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See also:

* https://civicrm.org/advisory/civi-sa-2019-15-xss-via-forged-mime-type
* https://lab.civicrm.org/dev/core/issues/1044