Dear Rabbi, I was wondering if there is an issue with putting a mirror in a tefillin bag?

ב"ה Shalom The Shulchan Aruch(או"ח סי' מב:ג ) actually specifically writes in regard to the use of the Tefillin bag for anything else besides Tefillin. When I say Tefillin bag, I refer to the actual bag in which the Tefillin are placed not the outer protective cover or over the shoulder bag. The Shulchan Aruch writes that if the Tefillin bag was set aside for the use of Tefillin only it may not be used for any other use, unless he had in mind to place in other things, such as a mirror in your case, he may do so. The Mishna Brura (ס"ק ט"ו )adds that things which are normally placed in a Tefillin bag such as a siddur, one cannot object to doing so since it is as if this condition for use of the Tefillin bag was already in mind when he bought it. It seems the same would apply to a mirror as well. All the best