copy pasting png file into RTE causes wrong content type (text/plain) being set on attachment

Summary

Ctrl+c ctrl+v of PNG file in RTE  causes wrong content type being set in fileattachment DB row for pasted file. This can cause not rendering images on certain browsers (IE) due to wrong content type returned by com.atlassian.jira.web.servlet.ViewAttachmentServlet

Environment

• JIRA 7.3.x

Steps to Reproduce

1. go to some issue
2. drag and drop ctrl+c ctrl+v some png file to the issue, it should be uploaded
3. copy the url from the uploaded file (from the view issue page)
4. use some tool to see headers of requested png file eq.
   curl -v -u admin:admin http://localhost:2990/jira/secure/attachment/10016/10016_image-2017-03-16-09-46-48-957.png > test
   gives:

< Content-Type: text/plain

where it should be 

< Content-Type: image/png

1. Also in db there is wrong content type set:
   image-2017-03-16-11-39-49-837.png
2. also when you copy paste the image url to new tab it will not render properly:
   image-2017-03-16-11-41-54-425.png

Expected Results

Content type should be set to proper one eg. image/png and image should be rendered correctly

Actual Results

Content type for copy pasted png files is set to text/plain

Workaround

use uploading from the menu (it seems to be working fine)