Fix API responses when dealing with txt files

changelogs/unreleased/issue_31790.yml

+---
+title: Fix API responses when dealing with txt files
+merge_request:
+author:

lib/api/files.rb

 module API
   class Files < Grape::API
+    # Prevents returning plain/text responses for files with .txt extension
+    after_validation { content_type "application/json" }
+
     helpers do
       def commit_params(attrs)
         {

spec/requests/api/files_spec.rb

@@ -33,6 +33,15 @@ describe API::Files do
         expect(Base64.decode64(json_response['content']).lines.first).to eq("require 'fileutils'\n")
       end
+      it 'returns json when file has txt extension' do
+        file_path = "bar%2Fbranch-test.txt"
+
+        get api(route(file_path), current_user), params
+
+        expect(response).to have_http_status(200)
+        expect(response.content_type).to eq('application/json')
+      end
+
       it 'returns file by commit sha' do
         # This file is deleted on HEAD
         file_path = "files%2Fjs%2Fcommit%2Ejs%2Ecoffee"
@@ -220,6 +229,7 @@ describe API::Files do
         post api(route("new_file_with_author%2Etxt"), user), valid_params
         expect(response).to have_http_status(201)
+        expect(response.content_type).to eq('application/json')
         last_commit = project.repository.commit.raw
         expect(last_commit.author_email).to eq(author_email)
         expect(last_commit.author_name).to eq(author_name)