Unable to upload attachments in the yahoo mail
My original answer got a downvote, despite being a valid suggestion. :P 

So, I'm going to try a different answer.

You are using a Xiaomi device, and you probably won't find a lot of Mi users here. They (MIUI) have their own forum, and you will probably find better support there.

The Mi browser you talk of is not available in Google Play. So, I'm willing to bet it's a custom browser packaged with the MIUI ROM, and I'm willing to bet it's not a very good one, given the problem you just described.

So, I'd suggest that you install a 3rd-party browser like Dolphin Browser. In the screenshot below, you can see that the image was attached even if Yahoo Mail was either in desktop mode (left) or in mobile mode (right).

Otherwise, just save yourself the trouble and just install the Yahoo Mail app.