I found something that might help you to identify your configuration. While checking the configuration Feature.FormsExtensions.AzureBlobFileStorageProviders.config
I found that you need to register the serviceType like this in your patch.
<services>
<register serviceType="Sitecore.ExperienceForms.Data.IFileStorageProvider, Sitecore.ExperienceForms"
implementationType="Feature.FormsExtensions.FileStorageProviders.AzureBlobStorageFileStorageProvider, Feature.FormsExtensions"
lifetime="Transient"
patch:instead="*[@serviceType='Sitecore.ExperienceForms.Data.IFileStorageProvider, Sitecore.ExperienceForms']" />
</services>
Check this link for the whole patch file.
https://github.com/bartverdonck/Sitecore-Forms-Extensions/blob/master/src/Feature/FormsExtensions/code/App_Config/Include/Feature/FormsExtensions/Feature.FormsExtensions.AzureBlobFileStorageProviders.config.disabled
Update:
You should also check that you have added the processor called AttachFileUploadProcessor
that is handling the logic to attach file to SendEmail action. Find the whole source code of this processor in this GitHub link.
https://github.com/bartverdonck/Sitecore-Forms-Extensions/blob/master/src/Feature/FormsExtensions/code/SubmitActions/SendEmail/FileAttachment/AttachFileUploadProcessor.cs
Also it should be in your config like this.
<pipelines>
<SendEmail role:require="Standalone or ContentManagement or DedicatedDispatch">
<processor patch:before="*[@type='Sitecore.EmailCampaign.Cm.Pipelines.SendEmail.SendEmail, Sitecore.EmailCampaign.Cm']"
type="Feature.FormsExtensions.SubmitActions.SendEmail.FileAttachment.AttachFileUploadProcessor, Feature.FormsExtensions" resolve="true" />
</SendEmail>
Check the whole config here with this link.
https://github.com/bartverdonck/Sitecore-Forms-Extensions/blob/master/src/Feature/FormsExtensions/code/App_Config/Include/Feature/FormsExtensions/Feature.FormsExtensions.Processors.config
Try this and hope this will solve your issue.
Thanks