Mime Type Problem with Country, State, provinces files on very old website v5.4.2.2

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A website I built almost six years ago and has been running well -http://museboutique.ie has been in touch with me regarding users not being able to submit orders due to the Address field not validating.

The Provinces list doesn't load in FF and in Chrome will load but won't go through. Site throws a validation error. Works fine in Safari.

I believe strict mime type checking is needed and the wrong ype is specified when trying to load these in. I need to change the mime type of a file somewhere but I can't seem to figure it out and don't want to break anything.

Has anyone encountered a similar issue or be able to help me solve this?

Thanks
Steve

C5ThemeTeam