Content type text/other is broken

**Describe the bug**
Switching Content Type "text/othe"r on Insomnia POST requests has no effect on Content Header.

**To Reproduce**
* Create a POST Request in Insomnia and select the Content Type (e.g. text/edn)
* Switch the Content Type to text/other
* Submit your request
* check Request timeline
* Content Type stays "text/edn"


**Expected behavior**
My expectation is that when i explicitely select the content header, there is a modification on submitted content header.
Option 1) clear the content header field in request
Option 2) because the option "other" is grouped under "text": set text/plain as Content-Header field

I prefer Option 2

**Screenshots**
see above

**Desktop (please complete the following information):**
- OS: Windows 10
- Installation Method: Traditional Autoupdate
- App Version 2021.03.0

**Additional context**
nothing

Thanks in advance

No source files or tests are named. Reproduce the issue by creating an Insomnia POST request, selecting a content type such as text/edn, switching to text/other, submitting it, and checking the request timeline. Done means the submitted Content-Type changes as specified when text/other is selected, preferably to text/plain.

Written by the indexing model from the issue text.