1. Do forms with text only (no attachments) send fine?
Yes
2. Are the image file extensions allowed in the mail tag?
Yes
3. How about the upload speed and final message when you upload a 4MB or 2MB image?
It works fine if I only upload 1 or 2 files, if I try more, it didn’t work
4. If you switch to the default WordPress theme is there any improvement in upload speed and success message?
No
5. With all plugins deactivated except CF7 is there any improvement in upload speed and success message?
No
After doing some different tests: the problem appears, if I upload more than 4 big files – however, this is necessary, I can not solve it otherwise …
Oliver