Manual:$wgMimeTypeBlacklist/de-formal

Details
Files with these MIME types will never be allowed as uploads if is enabled.

Default values

 * 'application/x-opc+zip', 'application/msword', 'application/vnd.ms-powerpoint', 'application/vnd.msexcel' were blacklisted after 1.17.0, and removed in
 * 'application/x-opc+zip', 'text/scriptlet', 'application/x-msdownload' were introduced after 1.5.5
 * 'application/x-msmetafile' was added in 1.5.5
 * 'application/zip' was added in 1.14 and then removed in
 * All other values were available since the setting was introduced in 1.5.0

Example
If you wanted to allow html files to be uploaded: