diff --git a/app/controllers/projects/blob_controller.rb b/app/controllers/projects/blob_controller.rb
index 7c65f1f5dfe9948c867309e684cbd91a4d3f09d1..a8c0a68fc17ccc97a41a1acde601399eb21a61a6 100644
--- a/app/controllers/projects/blob_controller.rb
+++ b/app/controllers/projects/blob_controller.rb
@@ -7,6 +7,7 @@ class Projects::BlobController < Projects::ApplicationController
 
   prepend_before_action :authenticate_user!, only: [:edit]
 
+  before_action :set_request_format, only: [:edit, :show, :update]
   before_action :require_non_empty_project, except: [:new, :create]
   before_action :authorize_download_code!
 
@@ -188,6 +189,18 @@ class Projects::BlobController < Projects::ApplicationController
       .last_for_path(@repository, @ref, @path).sha
   end
 
+  # In Rails 4.2 if params[:format] is empty, Rails set it to :html
+  # But since Rails 5.0 the framework now looks for an extension.
+  # E.g. for `blob/master/CHANGELOG.md` in Rails 4 the format would be `:html`, but in Rails 5 on it'd be `:md`
+  # This before_action explicitly sets the `:html` format for all requests unless `:format` is set by a client e.g. by JS for XHR requests.
+  def set_request_format
+    request.format = :html if set_request_format?
+  end
+
+  def set_request_format?
+    params[:id].present? && params[:format].blank? && request.format != "json"
+  end
+
   def show_html
     environment_params = @repository.branch_exists?(@ref) ? { ref: @ref } : { commit: @commit }
     @environment = EnvironmentsFinder.new(@project, current_user, environment_params).execute.last
diff --git a/changelogs/unreleased/blackst0ne-rails5-fix-blob-requests-format.yml b/changelogs/unreleased/blackst0ne-rails5-fix-blob-requests-format.yml
new file mode 100644
index 0000000000000000000000000000000000000000..a83aa03606a5770df23638e417d78893acfc7fd2
--- /dev/null
+++ b/changelogs/unreleased/blackst0ne-rails5-fix-blob-requests-format.yml
@@ -0,0 +1,5 @@
+---
+title: "[Rails5] Explicitly set request.format for blob_controller"
+merge_request: 19876
+author: "@blackst0ne"
+type: fixed