the file's extension, "png", does not match its underlying file type.

Hi Elmucho,

the reson is not really appian related.
Is you file really in the right format?
File format and extension are not the same.

"File extensions also often indicate the file type, or file format, of the file, but not always. Any file's extension can be renamed, but that won't convert the file to another format or change anything about the file other than this portion of its name."
-> for more information:
www.lifewire.com/what-is-a-file-extension-2625879

Hello Richard Michaelis I think the file format is correct since the file upload button on the dev environment is working. 

Hello Richard Michaelis this is the file upload button on the dev env. 

Hi elmucho,

check the admin settings for your environments.
i guess the difference is the following setting

Activated in your TEST environment but not in the DEV environment.
-> Can you check that?

I'll check on this sir Richard Michaelis thank you

That doesn't really tell you what the file's apparent underlying format is.  This can be a bit of a wild goose chase, but if you open the file in a (competent) text editor / hex editor, you might be able to check whether its metadata (usually the first few lines of text at the beginning of the file, which will look like garbage but contain some readable stuff) identifies it as a PNG or as some other image file type. 

If it really is a PNG (or you have strong reason to believe it is at least), you could alert Appian via a support case that the "block file not matching underlying type" feature Richard identified earlier is falsely flagging files.

For reference here's the first few lines of a PNG file opened in Notepad++

Versus the same from a .jpg file: