Catch and handle unallowed file type errors in issue attachment API (#30791) (#30834)

Backport #30791 by kemzeb

Before, we would just throw 500 if a user passes an attachment that is
not an allowed type. This commit catches this error and throws a 422
instead since this should be considered a validation error.

Co-authored-by: Kemal Zebari <60799661+kemzeb@users.noreply.github.com>

templates/swagger/v1_json.tmpl

@@ -7418,6 +7418,9 @@
           "404": {
             "$ref": "#/responses/error"
           },
+          "422": {
+            "$ref": "#/responses/validationError"
+          },
           "423": {
             "$ref": "#/responses/repoArchivedError"
           }
@@ -8037,6 +8040,9 @@
           "404": {
             "$ref": "#/responses/error"
           },
+          "422": {
+            "$ref": "#/responses/validationError"
+          },
           "423": {
             "$ref": "#/responses/repoArchivedError"
           }