Not sure if this is by design, but does seem like a small bug.
When uploading avatar flair image, it allows selecting any image file type. However, if you select the wrong one Discourse will let you know with an error.
This is because Discourses uses wildcard in the accept attribute:
<input class="hidden-upload-field" accept="image/*" type="file">
Yet, only a few file types are actually allowed to be uploaded. I uploaded an SVG image, which is not allowed. You can see what’s allowed to be selected and what Discourse actually allows:
Ideally, accept attribute should specify all allowed file types instead of a wildcard.
Discourse 2.9.0.beta3 (03ad88f2c2)