 <p>I ran into the same issue tonight, and it turned out I needed image/jpeg and image/jpg</p> <pre><code> validates_attachment :avatar, :styles =&gt; { :medium =&gt; "300x300", :thumb =&gt; "100x100" }, :content_type =&gt; { :content_type =&gt; "image/jpg", :content_type =&gt; "image/jpeg", :content_type =&gt; "image/png" }, :size =&gt; { :in =&gt; 0..1.megabytes } </code></pre> <p>It worked when just trying /^image/ but I like being specific.</p>