atproto.repoUploadBlob: allow parsing or specifying Content Type

[sodle]

When calling repoUploadBlob, the function always specifies the MIME Type as */*. This causes problems when trying to use the uploaded blob later, for example when specifying it as the avatar for a list:

XRPC ERROR 400: InvalidMimeType: Wrong type of file. It is */* but it must match image/png,image/jpeg.

[sodle]

Problematic code seems to be at repouploadBlob.go#L23.

Seems like it would be an easy fix to add a content-type parameter to this function -- however, the // Code generated by [...] DO NOT EDIT comment at the top of that file makes me hesitant to do that.

Additionally, a sensible default would be nice. How about:

• if a content-type arg is provided, use that
• if that arg is empty, attempt to detect the mime-type using something like vincent-petithory/dataurl
• if that also fails, default to */* as a last resort