Do not hiccup on unset Content-Type fields in multipart file uploads
Do not hiccup on unset Content-Type fields in multipart file uploads

Just don't set the content_type parameter in that case.

Fixes #985
(cherry picked from commit 4bb5be7)

mcanes/OctoPrint,foosel/OctoPrint,nicanor-romero/OctoPrint,beeverycreative/BEEweb,mcanes/OctoPrint,masterhou/OctoPrint,chriskoz/OctoPrint,dansantee/OctoPrint,eliasbakken/OctoPrint,nickverschoor/OctoPrint,Salandora/OctoPrint,CapnBry/OctoPrint,Salandora/OctoPrint,foosel/OctoPrint,eliasbakken/OctoPrint,leductan-nguyen/RaionPi,beeverycreative/BEEweb,mcanes/OctoPrint,nicanor-romero/OctoPrint,senttech/OctoPrint,Skeen/OctoPrint,foosel/OctoPrint,leductan-nguyen/RaionPi,punkkeks/OctoPrint,shohei/Octoprint,Skeen/OctoPrint,nickverschoor/OctoPrint,Javierma/OctoPrint-TFG,dragondgold/OctoPrint,dansantee/OctoPrint,beeverycreative/BEEweb,JackGavin13/octoprint-test-not-finished,eddieparker/OctoPrint,CapnBry/OctoPrint,MoonshineSG/OctoPrint,nickverschoor/OctoPrint,CapnBry/OctoPrint,Salandora/OctoPrint,ryanneufeld/OctoPrint,ErikDeBruijn/OctoPrint,MoonshineSG/OctoPrint,chriskoz/OctoPrint,Jaesin/OctoPrint,Mikk36/OctoPrint,Jaesin/OctoPrint,eddieparker/OctoPrint,chriskoz/OctoPrint,JackGavin13/octoprint-test-not-finished,dragondgold/OctoPrint,bicephale/OctoPrint,shohei/Octoprint,MoonshineSG/OctoPrint,beeverycreative/BEEweb,Javierma/OctoPrint-TFG,MaxOLydian/OctoPrint,dragondgold/OctoPrint,Jaesin/OctoPrint,MaxOLydian/OctoPrint,bicephale/OctoPrint,leductan-nguyen/RaionPi,nickverschoor/OctoPrint,leductan-nguyen/RaionPi,Salandora/OctoPrint,CapnBry/OctoPrint,masterhou/OctoPrint,senttech/OctoPrint,Mikk36/OctoPrint,ryanneufeld/OctoPrint,MoonshineSG/OctoPrint,bicephale/OctoPrint,JackGavin13/octoprint-test-not-finished,shohei/Octoprint,nicanor-romero/OctoPrint,ryanneufeld/OctoPrint,senttech/OctoPrint,shohei/Octoprint,JackGavin13/octoprint-test-not-finished,chriskoz/OctoPrint,Javierma/OctoPrint-TFG,ErikDeBruijn/OctoPrint,Skeen/OctoPrint,punkkeks/OctoPrint,MaxOLydian/OctoPrint,eliasbakken/OctoPrint,masterhou/OctoPrint,ErikDeBruijn/OctoPrint,Mikk36/OctoPrint,punkkeks/OctoPrint,shohei/Octoprint,ryanneufeld/OctoPrint,dansantee/OctoPrint,Jaesin/OctoPrint,bicephale/OctoPrint,Javierma/OctoPrint-TFG,eddieparker/OctoPrint,foosel/OctoPrint,senttech/OctoPrint