Are you sure you are using Adobe Acrobat Pro? What you describe is what would happen with the free Reader (unless you are using the latest version of the free Reader DC).
I am creating a form in Acrobat Pro (I have the Creative Cloud Service) so I can send to regular people that are using Reader to fill out the form what I want to do is use the Image field in the form editor witch I have but when I test it out in preview mode to test the fields the Image field does not allow me to select a .jpeg when it prompts me to browse for an image it only allows me to select .pdf or .ai everything els is dimmed and I can not select them?
When importing an image using Acrobat Pro, first you need to select the target file format in the local menu in the opening dialog.
(I know… Sometimes Acrobat's UI is strange…)