Uploading larger files failing with 50050

I am using the ConvertAPI to convert HEIC to JPG calling from Salesforce APEX. This works fine for smaller files but as the file gets larger (e.g. 4.1MB) I get a 50050 return code. Salesforce doesn't have builtin support for multipart but I have tried to workaround it by building my own multipart but that still returns the 50050.

The conversion works on the ConvertAPI website...I think because the upload may be separate from the Convert invocation (I just suspect that).

I do not want to have to do multiple api calls since I am already doing others to file the JPG after this call.

Any thoughts out there?