This is a classic problem! Buttons (and some other dynamic things) are set up by default to preempt other evaluations that may be underway but also to only assume that it will take a specific maximum amount of time. Generally when using SystemDialogInput and other things that will take longer than this, the action will time out and nothing then happens. This is all controlled by the Method Option to Button.

"With the default setting Method->"Preemptive", button actions are performed immediately, preempting any other evaluation, but are allocated only a limited time to complete."

So add

Method->"Queued"

to your Button and all should be well...

I had not seen the reply when I wrote this. I don't see a delete button, so moderator, please delete this post.

The following may not be very helpful, but it does update the filename. I also had trouble overriding the initialization, so I didn't initialize it.

Clear["Global`*"];

Manipulate[
Row[{Graphics@Disk[],Graphics[Text[Dynamic[filename],{0,0}],
 ImageSize->{200}]}],
 Button["Imagen?",filename=SystemDialogInput["FileOpen"]]]