What version are you running?
Gmail web client
What's the URL of the page containing the problem?
https://mail-attachment.googleusercontent.com/attachment/u/0/?ui=2&ik=d1199d5c39&attid=0.1&permmsgid=msg-a:r4028968924309220987&th=17d041c0dcfb77f3&view=att&disp=inline&realattid=f_kvrwjp8o0&ser=1&saddbat=ANGjdJ_f7VGvEt_-g7nU7ajs88hQLG0ETFF8AvC0xXqpkES4ARycxKN2DaNuBnkU5fU6OoqOFboCs2hqjcORtBMgA9LMmrKLQ4U1cKi9BGcdErzkfU_-RDR8emd1j5qobngH6wjEm8loefnwDkseQGm_3LzYRCX7LeH55dT0v5w1vyoGZpTWkHxhPNzy2w-mo0UTBppX8dgFjEpFPC69ag0lFzu9DkHYQU0mQjJxKSaiXyxDcURGcEz8keYU6C6VMjX3NzrdEsBA4XuR7aLfqgOP_v9L4NEHqjGNUD3_cfk_G8i6FMBvKJwg5jTCFXjmvgexkk-8bAym1cc1cGRpbNwPcxXqCbYFLT3QIUh55-wmA6rUTXof3B8GUj88OxHvoNDajWCXhSuUpXvi3T3bv30_E_hyUJRzCWX5RP8qScMj0l8IavO8c8UPLoSmT5OggCS9I4LCwNAyb8RPsFOZuOeQjj88mDAO_Xohvj5SOkjUduxlim4pXlho5Zq3XaFGg7QCz8oc8I_r37jse8B4J0hdcvEoFCb5-0IDy61ZuvGqoRLw0JbDYlq62iG-S9NhyUm-BUlgxSUHp1m-PrT2AyCaNE_RF9mWhTqtTN6cu5epWgdMJqDzIIsW-Nlagm0LMkVnZD__pji1qcNDfAjZlku7qtPKEDLryIEVDTLAQ37Mzx2KvTxRRrBb1hpLzKo
What steps will reproduce the problem?
1.Receive a html attachment .(this html inside has content-Disposition heaedr 'attachment ' 

Content-Type: application/octet-stream ; charset=UTF-8

Content-Transfer-Encoding: base64

Content-Disposition: attachment; filename=****.html)

clicking on the html attachmet in the received email .
Attachmet is not downloading raher opening directly in browser 

What is the expected output? What do you see instead?
Expected to download locally and  open.But html attachment is opening in new tab. 
What operating system are you using? What browser?
All browser are of same behaviour .reported a bug with chrome and its confirmed that its nothing to do with chrome. https://bugs.chromium.org/p/chromium/issues/detail?id=1262898#c12
Please provide any additional information below.
attached .html file have a content-disposition header  as 'attachment' .So expected to not open in browser rather download and open.Additionally , I observed view=att&disp=inline.

Yahoo the behaviour is as expected.only in gmail client we observe this issue.