Uploading an attachment with invalid mimetype displays an incorrect error message

Reproduction steps:

• Login as a user
• Go to a user profile
• Go the the page administration (http://localhost:8080/xwiki/bin/admin/XWiki/WebPreferences)
• In Content > Attachment Restriction, set a dummy allowed mimetype (e.g., abcdef)
• Go back to the user profile
• Upload a new attachment through the avatar attachment picker

Expected
An error message is displayed, informing the user that the mimetype of the attachment is not valid

Actual
The message {{The wiki administrators have defined a mimetype restriction for attached files. Please make sure the mimetype of the files you are trying to attach is allowed (0 bytes) and not blocked (

).}} is presented to the user in page http://localhost:8080/xwiki/bin/upload/XWiki/Admin