File extension changed after downloading a file from Issue page (BB-16050)

###Summary
When a user downloads a file on the Issues page, the file extension of the downloaded file changed. This issue only happens on Chrome.

###Tested Environment

• Hg and git repository
• Private Repository with private issue tracker
• Chrome (49.0.2623.112)

###Steps to Reproduce

1. Create an issue in the Issue Page
2. Attach files with different file extension
3. Click 'Create Issue'
4. Download each of the files

List of file extension tested:
(.sub, .vmg, .text, .ass, .asc, .xml, .log, .xlog, .ris, .klg, .conf, .java, .bin, .rar, .png, .xlsx)

###Expected Results###
The downloaded file should have the same file extension

###Actual Results###
The file extension changed to .txt when download .conf, .log and .asc file.
Some machine download file with .sql instead of .txt

###Workaround###

1. Use another browser (Firefox and Microsft Edge)
2. Right click on the file and click "Save link as...". Then change the file extension