An error occurred (InvalidParameterException) when calling the UpdateManagedLoginBranding operation: Media type not supported for value: text/plain

[CraigAtCatalyst]

When trying to add assets to aws cognito-idp update-managed-login-branding or aws cognito-idp create-managed-login-branding, I can't seem to find the proper way to upload the asset.

https://docs.aws.amazon.com/cli/latest/reference/cognito-idp/update-managed-login-branding.html says the file type should be "blob" and I've tried taking the asset output (byte string) from a "describe-managed-login-branding" request and additionally tried adding a file path as shown here: https://docs.aws.amazon.com/cli/latest/userguide/cli-usage-parameters-types.html#parameter-type-blob and seem to always get the error: An error occurred (InvalidParameterException) when calling the UpdateManagedLoginBranding operation: Media type not supported for value: text/plain

If I don't include assets in the command, everything works fine.

I've tried:

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets "$(< ./assets.json)"

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets '[{"Category": "FORM_LOGO", "ColorMode": "DARK", "Extension": "PNG", "Bytes": "file://logo.png"}]'

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets '[{"Category": "FORM_LOGO", "ColorMode": "DARK", "Extension": "PNG", "Bytes": "fileb://logo.png"}]'

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets '[{"Category": "FORM_LOGO", "ColorMode": "DARK", "Extension": "PNG", "Bytes": "./logo.png"}]'

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets '[{"Category": "FORM_LOGO", "ColorMode": "DARK", "Extension": "PNG", "Bytes": "$(cat ./logo.png | base64)"}]'

aws cognito-idp update-managed-login-branding --user-pool-id "USERPOOL" --managed-login-branding-id "BRANDING" --settings "$(< ./settings.json)" --assets '[{"Category": "FORM_LOGO", "ColorMode": "DARK", "Extension": "PNG", "Bytes": null}]' - which returned Invalid type for parameter Assets[0].Bytes, value: None, type: <class 'NoneType'>, valid types: <class 'bytes'>, <class 'bytearray'>, file-like object

[vgul]

I have the same issue

[uldyssian-sh]

That error happens because Bytes is a blob field, but when you put file://... or fileb://... inside JSON, AWS CLI treats it as a literal string and sends the text fileb://logo.png to the service.

Cognito then tries to interpret the “bytes” you sent and detects it as plain text (text/plain), which is why it rejects it with “Media type not supported”.

To make AWS CLI actually read the binary file and send it as a blob, you must pass the blob value via CLI argument parsing (where fileb:// is supported), not as a JSON string.

Use shorthand for --assets (this is the key):

aws cognito-idp update-managed-login-branding
--user-pool-id "USERPOOL"
--managed-login-branding-id "BRANDING"
--settings file://settings.json
--assets "Category=FORM_LOGO,ColorMode=DARK,Extension=PNG,Bytes=fileb://logo.png"

If you want to keep everything in JSON, then Bytes must be an actual Base64 string in the JSON (not fileb://...). For example, generate Base64 and put it into the JSON yourself:

BASE64=$(base64 -i logo.png | tr -d '\n')

Then in your JSON request:

{
"UserPoolId": "USERPOOL",
"ManagedLoginBrandingId": "BRANDING",
"Settings": { ... },
"Assets": [
{
"Category": "FORM_LOGO",
"ColorMode": "DARK",
"Extension": "PNG",
"Bytes": "BASE64_STRING_HERE"
}
]
}

Then call:

aws cognito-idp update-managed-login-branding --cli-input-json file://request.json

In short: fileb://... works for blob parameters when passed as an argument value, but it will not be expanded when embedded inside JSON; embedding it makes Cognito receive plain text, leading to the text/plain media type error.