I actually just figured it out. It was because my key wasn’t unique enough.
“mime_types” field is the field in the Code version for “Allowed file types” in the GUI of ACF, yes.
Also, as I described in my earlier post, I know that ACF doesn’t support SVG’s by default. I have added the support to WordPress myself. The images are upload-able to my media library. That is not the issue.
The issue is that when I have an Image Field within a Repeater, the image field completely ignores the mime_types that I’ve passed to it and will not allow me to select an SVG that is already uploaded to my media library. It tells me that the File type must be png, jpeg or jpg.
When I have an Image Field NOT within a Repeater, it respects the mime_types field and allows me to select an SVG image.
When you put an Image field within a repeater, it seems that it no longer supports the mime_types.