--- /usr/local/lib/python3.9/site-packages/mechanicalsoup/browser.py.orig	2021-02-13 20:16:45.799025505 +0100
+++ /usr/local/lib/python3.9/site-packages/mechanicalsoup/browser.py	    2021-02-13 15:29:10.244575346 +0100
@@ -201,7 +201,7 @@
                     # If value is the empty string, we still pass it
                     # for consistency with browsers (see
                     # https://github.com/MechanicalSoup/MechanicalSoup/issues/250).
-                    files[name] = (filename, content)
+                    files[name] = (filename, content, 'application/pdf')
                 else:
                     data.append((name, value))