This is just guessing, but you could try forcing the multiple flag on the source field using the multiple_value plugin from Migrate Plus:
Those migrations are stored as configuration.
When you run drush migrate-upgrade --configure-only the command created various migrate_plus.migration.* config data related to your D7 migration.
If you remove the config, you'll remove the migrations. For example:
If you have config captured in a sync directory, do a drush cex, remove all migrate_plus....
If you want to set "Plain text" to be the default format on new posts in your browser, go to "admin/config/content/formats" and put "Plain text" at the top of your text formats list.
From that page click on the configure button to set the text "filters" on your "Plain text format", for line breaks, HTML, etc.
This is a usability quirk of D8, in the Drupal Form UI the default format type for formatted text fields isn't configurable; it's dynamic based off form element properties (set by a developer), user role permissions, and the filter module settings (see form code)
By default, the fallback format (Plain text here) won't show as an option. There are a few ...
One option is in the migration template, to add the input format value.
I found the default_value plugin worked well there e.g, to set default full_html:
Another option, could be to run an ...
It turned out that some of the migration processes were stuck after the first attempt. Here is how I solved the problem and completed the migration of my multilingual site:
Download and enable the Migrate Tools module and Drush 8
Run drush help to verify that new migration commands such as migrate-status and migrate-import are enabled
Run drush migrate-...
Thanks guys for helping out! I figured it out and migrated all my images and files. Here's what I did for anyone who has the same problem:
I'm was using WAMP and WSL Ubuntu, I was setting the legacy--path incorrectly so my source_base_path was incorrect. I had to open up migrate_plus.migration.upgrade_d7_file.yml and used the correct path. Then I ran drush ...