propertiesView: Use decoder format

Use the format that we actually used when decoding.
The mime type is usually derived from the file extension
which might not always be correct.

Issue #43

Part-of: <https://gitlab.gnome.org/Incubator/loupe/-/merge_requests/132>

src/widgets/properties_view.rs

@@ -380,18 +380,6 @@ impl LpPropertiesView {
             .unwrap_or_else(|| FALLBACK.to_owned()) // and if we get nothing, use `FALLBACK`
     }
 
-    #[template_callback]
-    fn file_type(&self, info: Option<gio::FileInfo>) -> String {
-        info.and_then(|i| i.content_type())
-            .map(|t| t.to_string())
-            .map(|t| {
-                t.rsplit('/').collect::<Vec<&str>>()[0]
-                    .to_owned()
-                    .to_uppercase()
-            })
-            .unwrap_or_else(|| FALLBACK.to_owned())
-    }
-
     #[template_callback]
     fn file_size(&self, info: Option<gio::FileInfo>) -> String {
         info.map(|info| {