Slim3 supports File Upload.
See MultipartRequestHandler.java and FileUpload.java.
The uploaded result is stored in FileItem.java. So you can get it as follows:
The size of uploaded data is limited to 10MB. See Quotas_and_Limits.
The maximum entity size is 1MB. See Datastore Quotas_and_Limits.
If you want to upload more than 1MB, you should divide the uploaded data as follows:
See demo and source.
The above models use bidirectional one-to-many relationships. If you are not familiar with it, see relationships.
Continue to Validation.