PDFs aren't recognized unless the url ends in .pdf

[alexwykoff]

• Did you search for similar issues before submitting this one?
  yes

• Describe the issue you encountered:
  attempting to view http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.105.6357&rep=rep1&type=pdf in Brave fails, other browsers can view the pdf without issue

• Platform (Win7, 8, 10? macOS? Linux distro?):
  OS X

• Brave Version (revision SHA):
  0.15.2 (af7ef42)

• Steps to reproduce:

  1. visit http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.105.6357&rep=rep1&type=pdf pdf should be readable

• Actual result:

• Expected result:

• Will the steps above reproduce in a fresh profile? If not what other info can be added?
  yes

• Is this an issue in the currently released version?
  yes

• Can this issue be consistently reproduced?
  yes

• Extra QA steps:
  1.
  2.
  3.

• Screenshot if needed:

• Any related issues:

[bsclifton]

Closing in favor of brave/brave-browser#884